Natural History of the Bell Vireo, Vireo bellii Audubon

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Title: Natural History of the Bell Vireo, Vireo bellii Audubon
Author: Barlow, Jon C.
Language: English
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UNIVERSITY OF KANSAS PUBLICATIONS
MUSEUM OF NATURAL HISTORY

Volume 12, No. 5, pp. 241-296, 6 figs.
March 7, 1962

Natural History of the Bell Vireo,
Vireo bellii Audubon

BY
JON C. BARLOW

UNIVERSITY OF KANSAS
LAWRENCE
1962

UNIVERSITY OF KANSAS PUBLICATIONS, MUSEUM OF NATURAL HISTORY

Editors: E. Raymond Hall, Chairman, Theodore H. Eaton, Jr.,
Henry S. Fitch

Volume 12, No. 5, pp. 241-296, 6 figs.
Published March 7, 1962

UNIVERSITY OF KANSAS
Lawrence, Kansas

PRINTED BY
JEAN M. NEIBARGER, STATE PRINTER
TOPEKA, KANSAS
1962

29-1506

Natural History of the Bell Vireo,
Vireo bellii Audubon

JON C. BARLOW

CONTENTS

PAGE
CONTENTS 243
INTRODUCTION 245
ACKNOWLEDGMENTS 245
METHODS OF STUDY 246
STUDY AREA 247
Considerations of Habitat 248
SEASONAL MOVEMENT 250
Arrival in Spring 250
Fall Departure 251
GENERAL BEHAVIOR 252
Flight 252
Foraging and Food Habits 252
Bathing 253
VOCALIZATIONS 254
Singing Postures 255
Flight Song 255
Daily Frequency of Song 255
Types of Vocalizations 255
TERRITORIALITY 258
Establishment of Territory 259
Size of Territories 259
Permanence of Territories 260
Maintenance of Territory 260
Aggressive Behavior of the Female 264
Interspecific Relationships 264
Discussion 265
COURTSHIP BEHAVIOR 267
Displays and Postures 268
Discussion 270
SELECTION OF NEST-SITE AND NESTBUILDING 272
Building 274
Gathering of Nesting Materials 276
Length and Hours of Nestbuilding 277
Abortive Nestbuilding Efforts 277
Renesting 277
The Nest 277
EGGLAYING AND INCUBATION 278
Egglaying 278
Clutch-size 279
Incubation 280
The Roles of the Sexes in Incubation 280
Relief of Partners in Incubation 283
NESTLING PERIOD 283
Hatching Sequence 283
Development of the Nestlings 284
Parental Behavior 285
Feeding of the Nestlings 286
Nest Sanitation 287
Fledging 287
Nest Parasites 287
FLEDGLING LIFE 288
Second Broods 288
REPRODUCTIVE SUCCESS 289
Behavior 290
Predation 291
Cowbird Parasitism 291
SUMMARY 292
LITERATURE CITED 294

INTRODUCTION

The Bell Vireo (_Vireo bellii_ Aud.) is a summer resident in riparian
and second growth situations in the central United States south of
North Dakota. In the last two decades this bird has become fairly
common in western, and to a lesser extent in central, Indiana and
is apparently shifting its breeding range eastward in that state
(Mumford, 1952; Nolan, 1960). In northeastern Kansas the species
breeds commonly and occurs in most tracts of suitable habitat.

The literature contains several reports dealing exclusively with the
Bell Vireo, notably those of Bennett (1917), Nice (1929), Du Bois
(1940), Pitelka and Koestner (1942), Hensley (1950) and Mumford
(1952). Bent (1950) has summarized the information available on the
species through 1943. Nolan (1960) recently completed an extensive
report based on a small, banded population at Bloomington, Monroe
County, Indiana. He validated for this species many points of natural
history previously based on estimates and approximations, especially
concerning the post-fledging life of the young and the movement of the
adults from one "home range" to another in the course of a single
season.

None of these reports, however, has emphasized the ritualized
behavioral patterns associated with the maintenance of territory and
with courtship. Among the North American Vireonidae, the behavior of
the Red-eyed Vireo (_Vireo olivaceus_) is best documented (Sutton,
1949; Lawrence, 1953; Southern, 1958). With this species authors have
concentrated on the mechanics of the breeding season and their reports
contain little discussion of the aggressive and epigamic behavior of
the bird.

The amount of information on the ritualized behavior of the Bell Vireo
and related species heretofore has been meager. I observed breeding
behavior from its inception in early May through the summer of 1960.
It is hoped the resulting information will serve as a basis of
comparison in future studies of behavior of vireos; such ethological
data are becoming increasingly important, especially as an aid in
systematics.

ACKNOWLEDGMENTS

To professors Frank B. Cross, Henry S. Fitch, and Richard F. Johnston
of the Department of Zoology of the University of Kansas I am grateful
for comments and suggestions in various phases of the study and the
preparation of the manuscript. Professor Johnston also made available
data on the breeding of the Bell Vireo from the files of the Kansas
Ornithological Society. I am indebted to my wife, Judith Barlow, for
many hours of typing and copy reading. Mrs. Lorna Cordonnier prepared
the map, Thomas H. Swearingen drew the histograms, and Professor A. B.
Leonard photographed and developed the histograms. Dr. Robert M.
Mengel contributed the sketch of the Bell Vireo and George P. Young
prepared the dummy Bell Vireo used in the field work. Thomas R.
Barlow, Donald A. Distler, Abbot S. Gaunt, John L. Lenz, Gary L.
Packard, A. Wayne Wiens, and John Wellman assisted in various phases
of the field work.

METHODS OF STUDY

Daily observations were made from May 11 to June 26 in 1959 and from
April 15 through July 15 in 1960. Six additional visits were made to
the study area in September of 1959, and ten others in July and
August, 1960. Periods of from one hour to eleven hours were spent in
the field each day, and a total of about five hundred hours were
logged in the field.

Each territory was visited for at least five minutes each day but more
often for twenty minutes. The breeding activities of the pairs were
rarely synchronous. Consequently several stages in the cycle of
building were simultaneously available for study.

Nine young and one adult were banded in 1959. No Bell Vireos were
banded in 1960. Individual pairs could be recognized because of their
exclusive use of certain segments of the study area and by the
individual variation in the song of the males. Sexes were
distinguishable on the basis of differences in vocalizations and
plumages.

Most nests were located by the observer searching, watching a pair
engaged in building, or following a singing male until the increased
tempo of his song indicated proximity to a nest. As the season
progressed and the foliage grew more dense, it became increasingly
difficult to locate completed nests. Blinds were unnecessary because
of the density of vegetation. Observations were facilitated by a 7 x
50 binocular. Data were recorded on the spot in a field notebook. Eggs
were numbered by means of Higgins Engrossing ink as they were laid.

Individual trees in which males sang most were marked over a
three-week period. Then the distances between the most remote perches
were paced. These distances aided in determining the size of the
territories. The general configuration of the vegetation within each
territory determined the location of one or more boundaries of the
territory. Each territory was given a number, 1, 2, 3, etc., as it was
discovered; consequently there is no numerical relationship between
the designations of the territories established in 1959 and 1960.
Nests within a territory were designated as 1-a, 1-b, 1-c, etc.

Although experimentation was not a primary source of data, it proved
useful in certain instances. A stuffed Blue Jay elicited mobbing
behavior from nesting pairs. A dummy Bell Vireo elicited both
agonistic and epigamic behavior from nesting pairs, depending on the
phase of the nesting cycle.

The temperature at the beginning of each day's work was taken by means
of a Weston dial thermometer. A hand counter and a pocket watch having
a second hand were used in determining such data as frequency of song
and periods of attentiveness by the sexes. Histological
cross-sections, prepared by A. Wayne Wiens, of the ventral epidermis
of both sexes were used to study brood patches.

STUDY AREA

The intensive field work was on a 39-acre tract (fig. 1) extending
approximately 7/10 of a mile west from U. S. highway 59, which in
1959-1960 constituted the western city limit of Lawrence, Douglas
County, Kansas. The eastern boundary of the study area is
approximately 1-1/2 miles southwest of the County Courthouse in
Lawrence. The eastern ten acres is associated with the Laboratory of
Aquatic Biology of the University of Kansas. The 15 acres adjacent to
this on the southwest is owned by the University of Kansas Endowment
Association, but is used by Mr. E. H. Chamney for the grazing of
cattle. This portion is bounded on the west by a stone fence, beyond
which lies a 14-acre field of natural prairie owned by Dr. C. D. Clark
that is bordered on the extreme west by a narrow thicket of elm
saplings.

The principal topographic feature of the area is an arm of Mount
Oread, that rises some 80 feet above the surrounding countryside.
About 200 feet from the crest of the southwestern slope of the hill a
40-foot-wide diversion terrace directs run-off toward the two-acre
reservoir that is the source of water for eight experimental fish
ponds of the laboratory.

The predominant shrub-vegetation consists of Osage orange (_Maclura
pomifera_), honey locust (_Gleditsia triacanthos_), and American elm
(_Ulmus americana_). These saplings, ranging in height from 3 to 25
feet, grow in dense thickets as well as singly and in clumps of twos
and threes. Larger trees of these same species grow along the crest of
the hill, along the eastern and southeastern boundaries of the area,
and along the stone fence separating University land from that owned
by Dr. Clark. A dense growth of coralberry (_Symphoricarpos
orbiculatus_) forms the understory just below the crest of the hill.
Isolated clumps of dogwood (_Cornus drummondi_) and hawthorn
(_Crataegus mollis_) are scattered throughout the area. These species
of shrubs grow densely along the stone fence. The isolated thicket on
the Clark land is composed primarily of elm and boxelder (_Acer
negundo_), but includes scattered clumps of dogwood, Osage orange, and
honey locust. Poplars (_Populus deltoides_) are the only large trees
in this area.

[Illustration: FIG. 1. Map of the study area near the
University of Kansas Laboratory of Aquatic Biology. The dashed
lines mark the approximate territorial boundaries of the
original nine pairs of Bell Vireos from May 1960 to early June
1960.]

The open areas between the thickets are grown up in red top (_Triodia
flava_), bluestem (_Andropogon scoparius_), Switchgrass (_Panicum
virgatum_), Kentucky bluegrass (_Poa pratensis_), bush clover
(_Lespedeza capitata_) and mullen (_Verbascum thapsus_). Shrubby
vegetation occupies about 65 per cent of the total area, but in the
Clark portion constitutes only about 35 per cent of the ground cover.

_Considerations of Habitat_

Nolan (1960:226), summarizing the available information on habitat
preferences of the Bell Vireo, indicates that this species tolerates
"a rather wide range of differences in cover." He pointed out that a
significant factor in habitat selection by this species may be
avoidance of the White-eyed Vireo (_V. griseus_) where the two species
are sympatric.

In Douglas County where the Bell Vireo is the common species, the
White-eyed Vireo reaches the western extent of its known breeding
range in Kansas. At the Natural History Reservation of the University
of Kansas, where both species breed, the Bell Vireo occurs in "brush
thickets in open places" (Fitch, 1958:270) and the White-eyed Vireo
occupies "brush thickets, scrubby woodland and woodland edge" (Fitch,
_op. cit._, 268). Along the Missouri River in extreme northeastern
Kansas, Linsdale (1928:588-589) found the White-eyed Vireo "at the
edge of the timber on the bluff, and in small clearings in the
timber," while "the Bell Vireo was characteristic of the growths of
willow thickets on newly formed sand bars." Elsewhere in northeastern
Kansas I have found the Bell Vireo in shrubbery of varying density and
often in habitat indistinguishable from that occupied by White-eyed
Vireos at the Natural History Reservation. In the periphery of the
region of sympatry the rarer species is confronted with a much higher
population density of the common species and consequently might well
be limited primarily to habitat less suitable for the common species.
This would seem to be the case in eastern Kansas, presuming that
interspecific competition exists.

The Bell Vireo has followed the prairie peninsula into Indiana, aided
by the development of land for agriculture. In nearby Kentucky where
thousands of miles of forest edge are found, and where little brushy
habitat of the type preferred by the Bell Vireo occurs, the White-eyed
Vireo is abundant whereas the Bell Vireo is unknown as a breeding bird
(R. M. Mengel, personal communication).

In more central portions of the area of sympatry, nevertheless, the
two species do occur within the same habitat (Ridgway, 1889:191; Bent,
1950:254) and occasionally within the same thicket (Ridgway, in
Pitelka and Koestner, 1942:105); their morphological and behavioral
differences, although slight, probably minimize interspecific
conflict. The Bell Vireo and the Black-capped Vireo (_V.
atricapillus_) have been found nesting in the same tree in Oklahoma by
Bunker (1910:72); the nest of the black-cap was situated centrally and
that of the Bell Vireo peripherally in the tree. Bell Vireos
invariably place their nests in the outer portions of trees and
peripherally in thickets. This placement would further obviate
interspecific conflict with the white-eye since its nests are placed
centrally in the denser portions of a thicket.

A critical feature of the habitat preferred by the Bell Vireo is the
presence of water. In far western Kansas this species is restricted to
riparian growth along the more permanent waterways. This in itself is
not adequate proof of the significance of water supply because thicket
growth in that part of the state is found only along waterways. The 20
areas over the state that I have visited where Bell Vireos were
present were closely associated with at least a semi-permanent source
of water. Fifteen other areas indistinguishable from the 20 just
mentioned, but lacking a permanent supply of water, also lacked Bell
Vireos. Nevertheless areas in which Bell Vireos typically nest are
decidedly less mesic than those frequented by White-eyed Vireos.

Once the Bell Vireo was probably more local in its distribution being
restricted to thickets associated with permanent water. Clearing of
woodland for agricultural and other use, and subsequent encroachment
of second growth concomitant with the creation of man-made lakes and
ponds, has greatly increased the available habitat for this bird. The
preferred species of shrubs for nesting are reported (Bent, 1950:254)
to be various wild plums (_Prunus sp._). The widespread distribution
and abundance of the exotic Osage orange has greatly augmented the
supply of trees suitable for nesting.

SEASONAL MOVEMENT

_Arrival in Spring_

The subspecies of the Bell Vireo breeding in Kansas, _V. b. bellii_,
winters regularly from Guerrero and the Isthmus of Tehuantepec south
to Guatemala, El Salvador, and northern Nicaragua (A. O. U.
Check-list, Fifth Edition, 1957:469-470). In the United States
migrating birds are first recorded in early March (Cooke, 1909:119).
The Bell Vireo is a relatively slow migrator, moving primarily at
night and covering little more than 20 miles at a time (Cooke, _op.
cit._ 119). The average date of arrival, based on 27 records, for
northeastern Kansas is May 8; the earliest record is April 22, 1925,
from Manhattan, Riley County, Kansas (fig. 2-A).

In 1959 the first bird arrived at the study tract about May 5. No
additional birds were heard singing until the third week of the month,
in which eight new males were noted. As mentioned, in 1960 field work
was begun in mid-April and the study area was traversed daily. No
birds were detected until late afternoon of May 3, when one,
presumably a male, was seen foraging.

Lawrence (1953:50) has reported that males of the Red-eyed Vireo
precede females in the breeding area by as much as two weeks; the male
Red-eyed Vireo forages but sings little in the pre-nesting period. The
male Bell Vireo arrives first at the breeding area but precedes the
female by only a few days. On the morning of May 4 the first male was
singing from a number of perches while ranging over an area of seven
acres. This area encompassed territories later occupied by three
pairs, 2 (1960), 4 (1960), and 5 (1960). Late on the afternoon of May
4 the first courtship songs were heard and the first male was seen
with a mate at 6:20 p.m. Eight additional males arrived from May 6
through May 18. A tenth male was discovered in the vicinity of
territory 9 (1960) on June 18, 1960.

[Illustration: FIG. 2. Seasonal movement as indicated by the
curve for spring arrival (A), based on the earliest dates for
27 years, and the curve for autumn departure (B), based on the
latest dates for 21 years in northeastern Kansas.]

_Fall Departure_

The average date of departure for 21 years in northeastern Kansas is
September 3 (fig. 2-B). The earliest date is August 14 from Concordia,
Cloud County, Kansas (Porter, unpublished field notes). The latest
date is September 27 (Bent, 1950:262) from Onaga, Pottawatomie County,
Kansas. In 1959 the last vireo was seen at the study tract on
September 14. The birds do not all depart at the same time. On
September 1 there were still five singing males in the study area; by
September 10 there were three and on September 13, only one.

GENERAL BEHAVIOR

_Flight_

In "straight-away" flight the Bell Vireo undulates slightly. In a
typical flight several rapid, but shallow, wing beats precede a
fixed-wing glide of from 1 to 15 feet in length. Because the wings are
extended horizontally during the glide, the bird does not move
distinctly above or below the plane of flight. The White-eyed Vireo
generally appears to be slower and more lethargic in flight than the
Bell Vireo. In the breeding season most flights of the Bell Vireo are
no longer than a few feet between adjacent shrubs and trees, but
occasional sustained flights are as long as 300 feet. The birds fly as
low as 2 feet above ground, but have often been observed as high as 70
feet above the ground.

In courtship and protracted territorial disputes, where chase between
sexual partners or a pair of antagonists occurs, looping flights are
observed. The wings are beaten as the birds climb and many aerial
maneuvers are performed in the course of the glide.

_Foraging and Food Habits_

The Bell Vireo has been characterized as a thicket forager (Hamilton,
1958:311; Pitelka and Koestner, 1942:104), but in my experience it is
not restricted to low level strata; birds forage from ground level
upward, both in thickets and isolated trees ranging in height from 3
feet to 65 feet. The tendency to forage at higher levels is in part
dictated by the presence of tall trees within the various territories.

Territories 1 through 7 (1960) contained from three to ten trees
surpassing 40 feet in height. They grew singly or in small groves. The
Bell Vireos foraged fully 20 per cent of the time in these trees. Food
was sought throughout the leaf canopy.

Behavior in foraging in larger trees followed a routine pattern.
Typically a pair alighted in a tree at a height of 15 feet. Then the
female hopped to a perch a foot above the one upon which she landed.
The male succeeded her to the perch she had previously occupied. The
pair in effect spiraled around some large, essentially upright,
branch, in foraging. The birds usually reached higher perches in this
manner rather than by flying upward 10 to 15 feet to them. This manner
of progression within a tree is reminiscent of a similar habit of the
_Cyanocitta_ jays. Presumably, the habit of the Bell Vireo of foraging
in higher strata is facilitated by the absence of other species of
arboreal foraging vireos.

Chapin (1925:25) found the Bell Vireo to be more insectivorous in its
food habits than any other North American vireo. He found 99.3 per
cent of all food contained in 52 stomachs to be of animal origin. Only
three times have I seen a Bell Vireo take food of vegetable origin. On
September 9, 10, and 14, 1959, I noted a male eating wild cherries
over a period of 65 minutes of observation. Chapin (1925:27) noted
that beginning in July vegetable matter represented 1.57 per cent of
the bird's subsistence, and thereafter slightly more until fall
migration.

Animal food, consisting primarily of insects and spiders, is actively
sought along branches and under leaves. Often a foraging bird will
leap to the underside of a branch and hover, mothlike, beneath a
cluster of leaves while extracting some insect. Some individuals hung
upside down on small branches, paridlike, while foraging. Lawrence
(1953:710), and Southern (1958:201) have recorded similar behavior of
the Red-eyed Vireo. Occasionally, I have seen a Bell Vireo fly from a
perch and capture an insect in the manner of a flycatcher. The birds
do not appear to be adept at this type of food-getting. Nolan
(1960:242) mentions Bell Vireos holding hard-bodied insects by means
of their feet while breaking the exoskeleton with the beak to obtain
the soft parts. Southern (1958:201) recorded a female Red-eyed Vireo
foraging on the ground; I have seen a Bell Vireo on the ground but
once, and it was gathering nesting material.

_Bathing_

On May 14, 1960, in a rill that empties into the northeastern edge of
the reservoir a female flew down from a perch six inches above the
surface, barely dipped into the water, flew to a perch 12 inches above
the water, violently shook her ruffled body feathers, quivered her
wings, and rapidly flicked her fanned tail. The entire procedure was
repeated three times in five minutes. She was accompanied by a singing
male that did not bathe.

Nolan (1960:241) reports a male Bell Vireo bathing by rubbing against
leaves wet with dew; he notes that the White-eyed Vireo bathes in a
similar manner. Southern (1958:201) twice observed Red-eyed Vireos
bathing in water that dropped from wet leaves. In my study area in
1960, only territories 7, 8, 9, and 10 were not immediately adjacent
to permanent water. The pairs of Bell Vireos in those territories
presumably had to reply on wet vegetation for bathing.

VOCALIZATIONS

The male Bell Vireo begins to sing regularly soon after its arrival in
spring. Some daily singing continues following the cessation of
breeding activities until departure of the species in late summer or
early fall. The highest sustained rate of song occurs on the first and
second days of nest building. Because careful records of
meteorological data were not kept, I cannot significantly correlate
rates of song and specific temperatures and other weather conditions.
Frequency of song was reduced when the temperature rose above 90° F.,
as it did on many days in June, 1960. Nice (1929:17) mentions a
similar decrease in singing when the temperature exceeded 85° F.

Passerine birds typically sing at a high rate throughout courtship and
nestbuilding, but at a markedly lower rate thereafter. Most vireos are
atypical in this respect. In the study area in 1960 Bell Vireos sang
more often than Robins, Mockingbirds, Field Sparrows, Brown Thrashers,
Catbirds, and Doves breeding in the same habitat, about as often as
the Meadow Larks in the adjacent fields, and less often than Painted
Buntings.

The Bell Vireo seems to sing less often in the undisturbed state than
when aware of the presence of an observer. Observations from my car,
at a site approximately equidistant from territories 1 (1960), 2
(1960), 4 (1960), and 6 (1960) indicate that the rate of song during
incubation is decidedly less when no disturbing influence is present.
Normally, in this period, song aids in maintaining contact between the
members of a pair, serving to locate the male as he forages. Mumford
(1952:230) noted that the males often came out to meet him as he
entered their territories, singing as they approached. The male
typically continues to sing for some time after the intruder has
departed. Here the song acquires the additional functions of alerting
the female to danger and threatening the trespasser. Even after
allowance is made for this reaction to disturbance, Bell Vireos sing
more often than most of their nesting associates, and, on a seasonal
basis, they are vocal for a much longer time.

_Singing Postures_

In the normal singing posture the body of the Bell Vireo is maintained
at an angle of 35° to the horizontal. Occasionally, during nest
building, I have observed the body held at angles as severe as 80°
from the horizontal.

The head of the White-eyed Vireo is distinctly bobbed up and down, two
or three times, during the utterance of a song phrase. A bob involves
a deliberate withdrawal of the head towards the body and subsequent
sharp, almost vertical, extension of the neck. The head of the Bell
Vireo does not bob, although it vibrates as the song is delivered.

_Flight Song_

The Bell Vireo does not have a distinctive flight song; in fact, it
rarely sings or calls while in flight. Nolan (1960:240) has recorded a
male singing the normal song while in flight. Sharp scold-notes are
uttered in mid-air when a bird is agitated or actually attacking an
enemy. These notes and songs recorded by Nolan hardly qualify as
flight song, for this term implies use of a distinctive vocalization
not uttered in other circumstances.

_Daily Frequency of Song_

In the morning, Bell Vireos usually began singing a few minutes before
sunrise. Their songs were invariably preceded in the study area by
those of Western Kingbirds, Robins, Mourning Doves, Mockingbirds,
Cardinals and Meadow Larks. Bell Vireos sang relatively little after
6:30 p.m., even on the longest days of the year. The latest daytime
singing that was recorded was seven songs at 7:18 p.m. on June 20,
1960. A Cardinal in the vicinity sang for a full hour after this.

_Types of Vocalizations_

Six vocalizations were readily distinguishable in the field. These are
divisible into songs and call notes.

1. Primary song. It has been described by Pitelka and Koestner
(1942:103) as an "irregular series of harsh and sharp, but slurred
notes preceded by a few distinct notes of the same quality and ending
with a decided ascending or descending note of similar harshness." The
terminal note may also be somewhat abbreviated and intermediate
between an ascending or descending note. The song is sometimes
delivered as a couplet that consists of a phrase ending on a
descending note. This delivery is typical of incubation and later
renesting. During early season activities, the bird utters a phrase
ending on the descending note as many as 15 times before a phrase
ending on an ascending note is heard.

A sonagram of a single phrase, one of several recorded on May 9, 1960
(the third day of building of nest 1-b 1960), consists of 10 notes,
the first of which is distinct. The remaining notes are slurred. This
phrase is 1.4 seconds in length.

Songs are delivered most rapidly in the course of territorial disputes
and defense. The song is loudest in times of nestbuilding and periods
of aggressive behavior. At these times, on clear, calm days, the songs
are audible 100 yards away. Singing in the nestling period and
post-breeding season is audible at distances of no more than 50 feet;
such notes have been termed "whisper songs." Table 1 summarizes
singing rates at different periods of the nesting cycle in several
situations and under various weather conditions.

Songs are of equal frequency in the immediate vicinity of the nest and
elsewhere in the territory. Nice (1929:17) also found this to be true.
Perches can be almost at ground level or as high as 60 feet. Forty per
cent of my data on song concern singing at heights of more than 20
feet. As indicated in foraging, the lack of competition from aboreal
species of vireos presumably contributes to the use of higher perches
by Bell Vireos.

No female song was recorded in 1959, but on May 26, 1960, a female was
heard to sing once. She appeared at nest 1-f (1960) shortly after the
male arrived. Unlike him, she did not participate in building, but
seemed to be inspecting the nest. After 30 seconds she sang once--a
low garbled phrase--and also scolded once. After this she left. In the
meantime the continuously singing male moved two feet away from the
nest, then back to it and resumed construction.

The song of the female signaled to the male her departure. Pitelka and
Koestner (1942:103) heard a female sing twice after she replaced the
male on the nest. Females of three other species of vireos, the
Black-capped Vireo, _V. atricapillus_ (Lloyd, 1887:295), the
Philadelphia Vireo, _V. philadelphicus_ (Lewis, 1921:33), and the
Latimer Vireo, _V. latimeri_ (Spaulding _in_ Pitelka and Koestner,
1942:103) have been heard singing. Lewis and Spaulding also suggest
that the song of the female functions as a signal prior to exchange at
the nest.

The primary song identifies the singer as a male Bell Vireo. It
aids in securing a mate and in warning potential adversaries; also,
the song is a signal in certain situations and serves to locate the male.

TABLE 1. REPRESENTATIVE SINGING RATES OF BREEDING BELL VIREOS.
ALL RATES WERE AT AIR TEMPERATURES LESS THAN 86° F. EACH
INSTANCE REPRESENTS APPROXIMATELY 30 MINUTES OF OBSERVATION.

====================================================================
| | Average
Circumstance | Instances | rate per
| | minute
----------------------------------------------+-----------+---------
Attraction of mate | 2 | 6.3
Territorial dispute | 5 | 12.8
Nestbuilding | 6 | 7.0
Egglaying | 1 | 3.0
Incubation | 6 | 3.9
Exchange of partners in the incubation period | 1 | 4.0[A]
Foraging | 2 | 2.2
"Morning" song | 1 | 28.6[A]
"Evening" song | 1 | 1.9[A]
----------------------------------------------+-----------+---------
Over-all average rate per minute 6.3

[A] Not sustained; data representative of periods less than 5
minutes in length.

2. Courtship song. It is here termed the "congested" song and is
comparable to the adult "run-on" song mentioned by Nolan (1960:240).
The congested song is a squeaky version of the primary song and is
given when birds are engaged in pair-formation, nestbuilding, and
egglaying. The delivery is rapid and the sound can be likened to that
made by rapidly scraping a bow across a taut violin string. Nolan
(_in_ Mumford, 1952:230) is probably speaking of this song when he
describes a "tuneless" song that "had a jerky, sputtering quality that
characterizes part of the song of the Ruby-crowned Kinglet (_Regulus
calendula_)." More recently (1960:240) he applies the adjectives
"twanging," "Bobolink-like," "bubbling," "jerky," and "squeaky." This
song is often blended with the primary song and is audible for 75
feet.

A specialized version of the congested song is associated with
pre-and post-copulatory display but differs from the typical squeaky
performance in terminating in two ascending notes reminiscent of the
ascending phrase of the primary song.

3. Distress call. It was heard only once, when a captured bird was
being freed from a net. When the bird was almost disentangled it
uttered 10 high-pitched, plaintive notes. The quality of the notes
suggested a relationship to the song phrase rather than to other types
of vocalization. A nesting pair of Bell Vireos, 10 feet away, became
extremely excited when they heard the distress notes. They "scolded"
vigorously and flew around my head at a distance of six feet.

4. Alarm note. This is a specialized, three-note call of the male and
was heard only from the onset of pair-formation through early
nestbuilding. This whinnying, flickerlike call, phonetically
_eh-eH-EH_, each succeeding note of which is louder than the one
before, is given whenever the male is disturbed by an unfamiliar
object. This call is generally succeeded by the _chee_, but
occasionally blends into an extended "whinny," and is typically given
from some perch affording an unobstructed view of the offending
object. The male stretches his neck and cocks his head, the wings and
tail are not flicked or fanned, and no feather tracts are erected. The
bird, nevertheless, flits nervously from perch to perch when uttering
the call.

5. The _zip_. The male has a special "scold" note of his own that is
heard when an intruder first approaches the nest. Phonetically it is
_zip-zip-zip_. It is not so loud as the _chee_, and the delivery is
more deliberate than that note. If the intruder remains near the nest,
the _zip_ is usually replaced by the _chee_.

6. The generalized call note or _chee_. The call notes associated with
several situations are combined under this subheading since all can be
rendered in English by the same phonetic equivalent--_chee_. The
_chee_ associated with nestbuilding is of moderate pitch and delivered
deliberately at a rate of about 40 per minute. The feeding call of the
adults is a soft slurred _chee_, while that of the nestlings has a
mewing quality. In general, the _chee_ utilized in signal situations
consists of a few repetitions of the basic note emitted at a moderate
pitch. The _chee_ associated with hostile and courtship behavior is
higher pitched and the delivery is much more rapid, approximately 200
per minute. Nolan (1960:240) reports a continuous rate of 25 per five
seconds when an adult Bell Vireo is alarmed. The _chee_ of extreme
anxiety is a loud emphatic buzz, phonetically ZZ-ZZ-ZZ-ZZ.

TERRITORIALITY

The Bell Vireo exhibits "classic" passerine territoriality. Within a
specific area, a pair of this species carries out pair-formation,
courtship activities, copulation, nesting, rearing the young, and
foraging. With the cessation of reproductive activities, a pair
continues to restrict its other daily activities to the same general
area.

_Establishment of Territory_

In early May the segment of the total suitable habitat within which a
Bell Vireo restricts its activities is not rigidly defined and the
first male of the season ranges over an area too large to be
maintained permanently--one that seems greatly to exceed the needs of
breeding. Male 1 (1960), for instance, was first seen foraging over an
area of approximately seven acres. With the influx of other males,
portions of this large tract were usurped and the territory of the
original male was gradually reduced to an area of little more than an
acre.

In this initial period, a male becomes identified with a large area
but is restricted to an area of nearly typical size by the
encroachment of other males. Territorial disputes in this period often
involve physical contact, as well as protracted sessions of
high-intensity singing at rates exceeding three hundred song-phrases
per hour.

Eventually the carrying capacity of the habitat is reached and no
further partitioning occurs. The beginning of nestbuilding coincides
with this relative stabilization of the territorial boundaries.
Through the remainder of the cycle of behavior associated with any one
nest, all activity is that of the occupant pair within its territory.

_Size of Territories_

The nine original territories established in 1960 varied in size from
0.26 acre to 3.1 acres (Table 2). Fitch (1958:270) found the
territories of several pairs of Bell Vireos at the University of
Kansas Natural History Reservation to vary from 0.4 to 1 acre. Hensley
(1950:243) estimated the size of the territory of a pair of Bell
Vireos observed in Piatt County, Illinois, at 3.1 acres. Nolan
(1960:227) records home ranges of 2 to 3 acres. The pairs that he
studied were sole occupants of fields several acres larger than the
portions actually utilized. His description of the vegetation
indicates that most of the second growth was not much taller than 7
feet. As indicated elsewhere, the second-growth in my tract averaged
15 feet tall. The smaller average size of territory (1.25 acres) that
I found is probably a function both of this greater vertical range of
available foraging area and the much higher gross density of birds (40
pairs per 100 acres).

_Permanence of Territories_

Most pairs remain in their original territories throughout the summer,
although some shift certain territorial boundaries. In 1960 pairs 2
and 6, in the course of selecting a site for a replacement nest,
annexed adjacent areas previously occupied by other pairs. Pair 2
relocated in a space that originally included territories 1 and 4, and
pair 6 built a nest in an area formerly occupied by pair 7. Males 1
and 4 were sacrificed for specimens and pair 7 probably was destroyed
by a predator. Owing to the presence of a nest, the annexed area
becomes the focal point of the activities of a pair, but the original
area is regularly visited and may be returned to in a later renesting.

TABLE 2. SIZE OF THE NINE ORIGINAL TERRITORIES OCCUPIED IN 1960.

==========================================
| Date first |
Territory | occupied | Dimensions
-------------+--------------+-------------
1. | May 3, 1960 | 1.6 acres
2. | May 5, 1960 | 0.6 acre
3. | May 7, 1960 | 0.26 acre
4. | May 11, 1960 | 1.03 acres
5. | May 12, 1960 | 2.07 acres
6. | May 14, 1960 | 3.1 acres
7. | May 13, 1960 | 1.7 acres
8. | May 14, 1960 | 0.46 acre
9. | May 14, 1960 | 0.4 acre
-------------+--------------+-------------
Average 1.25 acres

_Maintenance of Territory_

Except in the early stages of nesting, territory is maintained
primarily by song. In the period of incubation a male regularly
patrols his territory between sessions of sitting on the eggs. He
sings several songs from each of several perches. A male follows a
predictable path, rarely traveling more than 150 feet from the nest.
Incipient patrolling is seen early in the breeding season when
territorial boundaries are in a state of flux.

The male White-eyed Vireo travels a semi-predictable route, as does
the Solitary Vireo (R. F. Johnston, MS). According to Lawrence
(1953:50), the male Red-eyed Vireo has a distinct singing area
completely divorced from the nest area dominated by the female.
Southern (1958:109), working with this same species in Michigan, did
not recognize separate areas, but found that the male wandered
randomly over the territory.

In a species so highly active as the Bell Vireo, the degrees of
hostile action associated with an encounter overlap in such a fashion
that no clearcut distinction can be drawn among the various displays.
Nevertheless, certain generalized patterns are characteristic of all
situations in which members of this species are in a state of anxiety.
The threat displays described in the succeeding paragraphs may all be
utilized within as little as two minutes; mutual agonism may be
terminated at any stage by concerted attack of the dominant bird.

1. Vocal threat. When an intruder is discovered the resident male
markedly increases his rate of singing. The alarm note, _eh-eH-EH_, is
the first call uttered during the nestbuilding and egglaying periods.

2. Head-forward threat. If the intruder does not flee, the resident
male adopts a specific threat posture. The head and neck are extended.
The feathers of the crown are erected, but those of the body are
sleeked. The bird crouches slightly and the tail is flicked laterally,
but not fanned. The intensity of the singing increases and is
supplemented by scolding, also delivered at a rapid rate. The intruder
normally retreats at this juncture.

3. Wing-flicking and submaximal tail-fanning. If the interloper
remains, the anxiety of the resident male increases. He slightly
depresses the tail and, at the same time, rapidly fans and closes it.
The tail is only partially fanned. The wings are held slightly away
from the body and rapidly flicked above the back. This flicking should
not be confused with quivering of the wings associated with begging
and other solicitory postures. Song is now almost completely replaced
by high-intensity scolding. Associated with this high degree of
anxiety are displacement behaviorisms, including bill-wiping, reversal
of direction on a single perch, and a nervous hopping from one perch
to another.

4. Ruffling and maximum tail-fanning. This display is most often seen
in conjunction with the harassment of predators, but occasionally it
is observed in territorial disputes occurring at the boundary of
adjacent territories where neither male is strictly dominant and in
which there is much vacillation prior to attack. The feathers of the
abdomen are ruffled. The term "ruffled" pertains to a full erection of
the feathers, giving a ragged appearance to the body outline (Morris,
1956:80). Ruffling of the abdominal feathers emphasizes their yellow
color and seemingly heightens the intimidatory effect. The tail is
fully fanned, and so maintained, for a few seconds at a time; it is
held at a 45° angle to the body. The scold becomes an extremely
intense, stacatto buzz, ZZ-ZZ-ZZ-ZZ.

5. Supplanting attack. The attack directed against a trespassing male
is initiated as a lunge that results in a collision with the opponent
in mid-air or on his perch. The bird attacked is struck by his
adversary's open beak or body.

Hinde (1952:71-72) indicates four courses of action followed by a
Great Tit (_Parus major_) when attacked under similar circumstances.
"(a) It flies away: The attacker usually flies after it and a chase
ensues. (b) It shifts its perch a few inches: the attacker lands in
its place, and both usually show head-up postures. (c) It remains
where it is, but adopts a head-up posture: the attacker usually then
shows upright flight. (d) It may fly up and meet the attacker in
mid-air: in that case an actual combat may result, or both combatants
may show upright flight."

Head-up posturing and upright flight are not presently recognized
components of the behavior of the Bell Vireo. The behavior of the
attacked Bell Vireo is similar to that described in (a), (b), and (d)
above, and is clearly dictated by the proximity of his own "home
base."

Eleven disputes among occupants of adjacent territories were witnessed
between May 6 and June 3, 1960, in which some or all of the described
threat displays were manifest (Table 3). In each instance, patrolling
males were gradually attracted to each other. As they approached,
their rates of song increased from an average of six repetitions per
minute to 15 per minute. Eight of the disputes involved physical
combat.

On May 6, 1960, when male 2 (1960) was in the process of usurping an
eastern segment of the original territory of male 1 (1960), a violent,
protracted dispute was observed. By this date male 1 (1960) had
obtained a mate and had begun construction of nest 1-a (1960); male 2
(1960) had not yet acquired a mate. At first the two males were
singing vigorously, from one to 10 feet apart. Female 1 (1960)
followed her mate closely and scolded, at the same time partially
fanning her tail. In the course of vocal dueling the males had
traveled to within 50 feet of nest 1-a (1960), when male 1 (1960)
suddenly lunged at 2 (1960). The males plunged to the ground, locking
bills and clutching at each other with their feet as they fell. As
soon as they touched the ground they separated. Male 2 flew east with
male 1 in pursuit. This conflict lasted three minutes.

Additional physical combat was witnessed several minutes later. This
again involved striking with the bill, wings and feet. A high pitched
squeaky _chee_ was uttered by both combatants. The female scolded from
a nearby perch. Upon separating, the males engaged in a wild, looping
flight. At about 350 feet from nest 1-a (1960), the chase abruptly
ended. For ten minutes thereafter, both males sang at a high rate from
perches about 10 feet apart. This terminated the physical combat, but
three additional protracted, vocal duels occurred in the remainder of
the morning.

TABLE 3. INTRASPECIFIC DISPUTES IN MAINTENANCE OF TERRITORY.

Behavior

==============================================================
| Number | | | Average
| of | Vocal | | length of
| conflicts | dueling | Combat | disputes
-------------+-----------+---------+--------+-----------------
Prenesting | 3 | 3 | 2 | 6 min. 40 sec.
Building | 8 | 8 | 6 | 3 min. 8 sec.
Incubation | 1[B] | 1 | ... | 20 min.
+-----------+---------+--------+-----------------
Totals | 12 | 12 | 8 | 5 min. 30 sec.
-------------+-----------+---------+--------+-----------------

[B] Directed against a stuffed Bell Vireo.

Probably as a direct result of these conflicts, a neutral zone
approximately 300 feet wide developed between the two territories. By
May 14 this intervening area was occupied by male 4 (1960). By this
date both 1 (1960) and 2 (1960) were involved in nestbuilding and 4
(1960) was not challenged for several days.

Maximum tail-fanning prior to attack also appears as an element of
aggressive behavior in White-eyed Vireos. A brief skirmish between a
male of this species and a small, greenish passerine was observed at
the Natural History Reservation on May 25, 1960. The White-eyed Vireo
was singing from a perch 30 feet high in a dead elm, when the
unidentified passerine landed 10 feet distant. The white-eye ceased
regular song and uttered several catbirdlike calls, and at the same
time slightly depressed and fully fanned the tail. After 10 seconds,
the white-eye lunged at the intruder, striking it in mid-air. A brief
looping flight ensued through the branches of the elm before the
intruder was able effectively to retreat.

_Aggressive Behavior of the Female_

The female Bell Vireo is concerned primarily with the defense of the
nest and the young and she rarely assists the male in defense of
distant parts of the territory. She employs the same threat displays
as the male.

_Interspecific Relationships_

A number of meetings between Bell Vireos and other species were
observed in the course of the study (Table 4). Resident pairs of this
species exhibited different degrees of tolerance toward other species.
Many birds, including Cardinals, Field Sparrows, Painted Buntings and
Mourning Doves were ignored completely. Chickadees evoked responses
characterized by slight increase in song and some anxiety; this was
perhaps owing to similarity in size, motion and call notes. Warblers,
when met with, were invariably chased. They may be momentarily
mistaken for rival vireos.

TABLE 4. INTERSPECIFIC CONFLICT OBSERVED IN 1959 AND 1960.

==================================================================
| Number | Phase of | Behavior of
| | | Bell Vireos
Species | of | breeding +------+---+----+---
|conflicts| cycle |HFT[C]| S | TF | A
---------------------+---------+--------------+------+---+----+---
_Coccyzus | 1 | Nestling | | | |
americanus_ | | period | -- | x | |
| | | | | |
_Cyanocitta | 3[D] | Nestling and | | | |
cristata_ | | incubation | | | |
| | period | x | x | x | x
| | | | | |
_Parus atricapillus_ | 1 | Prenesting | -- | x | |
| | | | | |
_Molothrus ater_ | 1 | Nestling | | | |
| | period | -- | x | -- | x
| | | | | |
_Dendroica petechia_ | 1 | Prenesting | -- | x | -- | x
| | | | | |
_Geothlypis trichas_ | 1 | Nestbuilding | -- | x | -- | x
| | | | | |
_Pituophis | | | | | |
catenifer_[E] | 1 | Post-fledging| -- | x | -- | x
---------------------+---------+--------------+------+---+----+---

[C] HFT = head-forward threat; S = scolding;
TF = tail-fanning; A = attack.

[D] Includes attack against a dummy Blue Jay.

[E] The Bull Snake is here included because the vireos
directed typical aggressive displays towards it.

Blue Jays were vigorously attacked, especially late in incubation and
throughout the nestling period of the Bell Vireo. I did not see a jay
struck, but a vireo would circle one closely as it perched and pursue
it when it flew, following as far as 100 yards beyond territorial
bounds. The buzz, ZZ-ZZ-ZZ-ZZ, was uttered in conjunction with this
harassment.

A stuffed jay placed eight feet from a nest elicited threat display
and displacement behavior from the owners of the nest, but no attack.
Incubation had just begun at this nest. A dummy Bell Vireo placed
close to another nest only momentarily disturbed the male, and the
female completely ignored it. Incubation had also recently begun at
this nest. At this same general stage, moreover, nesting pairs showed
little inclination to harass me.

_Discussion_

Hinde (1956:341-342) indicates that territory has been defined in a
number of ways by many workers. All of the definitions involve
modification of Howard's classic "defended area." Pitelka (1959:253)
has reacted against this behaviorally-oriented concept. He thinks that
the concept of territory should be based on exclusive use of an area
by its occupants, and not so much the defense by which they maintain
it.

Methods of treating territoriality in the Bell Vireo seemingly
incorporate features of both schools of thought. The area used
exclusively for all biological needs by a single pair of Bell Vireos
is vigorously defended both physically and vocally early in the
breeding season and vocally as the season progresses.

In the period of territorial establishment a relatively large area is
actively defended. The building of a nest establishes a focal point of
activity in a somewhat more restricted area than that originally
occupied. After the success or failure of a nest, a new site is
selected to which the focal point of activity is shifted. If suitable
habitat adjacent to the extant territory is unoccupied by other Bell
Vireos the unoccupied area may be annexed in the course of searching
for a new site. Such annexation occurs only when pairs formerly
occupying adjacent suitable habitat disappear from this territory;
possibly the size of the territory of any one pair is dictated by the
density of population of the species as well as by the presence of
suitable habitat. This may not always be true as indicated by Kliujver
(1951:40), who in studying the Great Tit, found no appreciable
difference in the size of territory in two different habitats even
though there was a marked difference in population density of the
birds.

Fluctuation of territorial boundaries is not uncommon in passerines,
especially when no rivals exist to contest movement. Hinde (1956:351)
indicates that fluctuations in size of territory are to be expected
although the territories of different species of birds have different
mean sizes.

Once nesting activities commence there is a marked reduction in the
amount of territory utilized and a distinct decrease in the aggressive
tendencies of the male; it would seem that energy previously utilized
in regular fighting is rechanneled for nestbuilding, incubation and
care of the young. Further, contraction of the area of activity
obviates high-intensity territorial defense, as adjacent males, even
in regions of high population density, are isolated from one another
by an area no longer regularly traversed.

With cessation of breeding activities physiological mechanisms
governing maintenance of territory seemingly are no longer active and
yet the pairs of Bell Vireos remain within a restricted area which
they alone use. Earlier definitions of territory as a "defended area"
do not adequately cover such situations and yet from the standpoint of
Pitelka the area still retains the characteristics of true territory.
In fact, territory as defined by Pitelka is clearly manifest at this
time. Whether the birds remain in an area through "force of habit" is
of little consequence.

I have retained the term "territory" in preference to the term "home
range" used by Nolan (1960:227). His failure to observe territorial
defense is responsible for his terminology, although it is readily
understandable that such defense would be lacking in a population of
relatively low density in which pairs were isolated from one another
by areas of unfavorable habitat. This isolation in itself would tend
to preclude territorial conflict but territories were, in fact,
maintained.

The marked similarity in the essential features of aggressive behavior
in North American vireos attests to their close relationship. Flicking
and fanning of the tail are distinct components of the hostile
behavior of the Bell Vireo, White-eyed Vireo, Red-eyed Vireo
(Lawrence, 1953:69), and the Black-whiskered Vireo (_Vireo
altiloquus_; Bent, 1950:319), and, presumably, of the remaining
species of the genus. The occurrence of these same displays as
intrinsic behavioral elements of interspecific hostility suggests a
common derivation. Moynihan (1955:256) indicates that all
intraspecific hostile displays, and probably most interspecific
hostile displays, evolved originally as social signals having the same
general function. Further, Hinde (1956:344) points out that there is a
fundamental similarity in the motor patterns used in fighting in
different contexts, including both interspecific and intraspecific
fighting.

COURTSHIP BEHAVIOR

The precise mechanism of pair-formation in the Bell Vireo is not
known. My experience has been to find a male one day and then one or
two days later to discover that it has a mate. Lawrence (1953:53),
tells of a male Red-eyed Vireo singling out a female from a flock of
migrants passing through his territory and violently driving her to
the ground. Shortly after this attack the pair was seen searching for
a nest site. But such an incident has not been reported for other
vireos, nor have I witnessed such behavior myself.

Early courtship activities of the Bell Vireo are characteristically
violent affairs, with the male directing strong aggressive attacks
toward the female. Rapid, looping flights through the thickets occur,
the female leading the male. Occasionally he deliberately collides
with her in mid-air, but the pair quickly separate. This violent
sexual chasing is manifest prior to the inception of nestbuilding.
With commencement of this activity, sexual chases through the
territory subside.

Absence of sexual dimorphism in the Bell Vireo obviously suggests that
behavioral criteria are used by the birds in sex-recognition. The lack
of aggression by the female upon initial aggression by the male is an
essential component of recognition of sex; she is clearly subordinate.
Such subordination is also the significant feature of continued
sex-recognition. Courtship display by a resident male, directed toward
a stuffed male and a wounded male which sat motionless, supports the
contention that a subordinate or submissive attitude of the female is
a key factor in sex-determination.

Nestbuilding and courtship are intimately associated in this species.
The male constructs the suspension apparatus of the nest, the
completion of which coincides with the assumption of nestbuilding
activity by the female. Roles of the sexes in nestbuilding are
described in the section on nestbuilding. The male frequently
interrupts construction to court the female. This, in combination with
perpetual song as he works, serves to strengthen the pair-bond and
stimulate nestbuilding tendencies of the female.

It is doubtful that any attempts at copulation are successful up to
this time. The female is singularly unresponsive to the advances of
the male; a female retreats before most violent attacks and is
seemingly oblivious to less vigorous behavior. After the female
assumes the responsibility of building, the tempo of courtship
activities increases.

The female becomes increasingly more receptive and her work is often
interrupted by advances of the male. Copulation occurs frequently from
about the third day of nestbuilding through the first day of
egglaying, a period of four to six days. Male displays and
vocalizations associated with courtship continue through the fourth or
fifth day of incubation.

_Displays and Postures_

The principal courtship displays and postures that were seen
throughout the nestbuilding phase are as follows:

1. Greeting ceremonies. Both birds are crouched from one to five
inches apart. The feathers on one (the male?) are sleeked, and on the
other are fluffed. Fluffing (Morris, 1956:80) denotes partial erection
of the body feathers producing a rounded, unbroken body line and is
not to be confused with ruffling, mentioned in the sections pertaining
to territoriality and pre- and post-copulatory display. Fluffing is
generally considered to be an appeasement display and it is seen in a
variety of situations involving a dominant-subordinate relationship.
Both birds flick wings and tails rapidly and reverse directions on
their perches frequently. A low, rapid _chee_ is uttered during this
performance. This ceremony is repeated often in the first three days
of nestbuilding, but less frequently thereafter. It usually occurs
after building by one or both partners and prior to another trip in
search of nesting material. It lasts from 10 to 50 seconds and is not
immediately followed by any additional courtship activities. Nolan
(1960:228-229) observed mutual displays between periods of violent
sexual chase that suggest that the greeting ceremonies that I have
described are an integral part of pair-formation as well as a
component of continued maintenance of the bond.

2. "Pouncing." The female rapidly quarter-fans and partially depresses
her tail. She utters a high pitched scold (_chee_). The male, from a
perch within two feet of the female, fans the tail fully and depresses
it vertically, and, with mouth open, lunges at the female; or, with
similar tail mannerisms, the abdominal feathers ruffled, the wings
held horizontally, and the primaries spread, he sways from side to
side from four to six times, and then lunges at the female. The male
is silent when he pounces; the _chee_ or the courtship song is emitted
when swaying precedes pouncing. The male strikes the female with his
breast or with his open beak. The female rarely flees although she is
usually displaced several inches along the branch upon which she is
sitting. However, the female may fly several inches to a new perch.
The failure of the female to adopt a solicitation posture presumably
indicates sexual unreadiness. Instances of the male deliberately
colliding with the female as she flies in the course of gathering
nesting material are probably analogous to pouncing. In none of the
above situations are females observed to fight back in any way. Nice
(1943:174) believed pouncing to be analogous to sexual chasing found
in such species as the Red-winged Blackbird. In the Song Sparrow,
pouncing is observed most often in the first and second days of
nestbuilding.

3. "Leap-flutter." The male, in the course of displaying with the tail
fanned before the female, suddenly leaps eight inches to ten inches
vertically and flutters in mid-air several seconds, before dropping to
the original perch. This display occurs in full view of the female. It
is often associated with pouncing and is also seen prior to
copulation. In the latter instance it is probably pragmatically
functional, for it permits the male to orient above the female before
dropping to her back to copulate. No vocalization is uttered during
the leap-flutter.

4. Pre-copulatory display (Fig. 3). The male faces the female. The
tail is fanned fully and depressed at a sharp vertical angle to the
body. Body feathers, both dorsal and ventral, are ruffled, almost
tripling the apparent volume of the thorax. The head is withdrawn and
slightly thrown back. Feathers of the head are not erected. The mouth
is opened wide. The legs are slightly flexed and the body is swayed
laterally. Horizontally, the head and body traverse an arc of about
100°; vertically, they traverse an arc slightly less than 180°. At the
low point of any one swing, the delivery of the courtship song begins.
At the termination of the swing the two normal, ascending notes are
emitted. This performance may last as long as three minutes.

[Illustration: FIG. 3. A single male Bell Vireo in the
pre-copulatory display. Note the ruffled dorsal and ventral
body feathers. The male on the left has reached the zenith of
a single swing. The male on the right has nearly reached the
low point of a swing.]

The pre-copulatory display of the male elicits receptive behavior in
the female. She crouches in a solicitous manner, with the body
feathers fluffed and the tail raised slightly, and utters a muted
_chee_.

5. Copulation. The male abruptly terminates his swaying display with a
leap-flutter that positions him above the female's back. He then
descends and copulation occurs. The male continues to flutter his
wings to maintain balance throughout the two seconds of cloacal
contact. Following an unsuccessful copulation on June 23, 1960,
displacement preening and bill wiping were performed by both sexes.

6. Post-copulatory display. On June 25, 1960, after a second attempt
at copulation with a stuffed bird in which semen was actually
deposited on the dummy's back, male 10 (1960) performed a swaying
display. In this instance, however, instead of addressing the dummy
from the front, the male alighted one inch to the right of the stuffed
bird. When swaying to the left (toward the dummy) the head of the
displaying male actually passed above the neck of the stuffed bird.
This ritualized behavior could conceivably be derived from
hetero-preening.

_Discussion_

Within the scope of my research it was difficult to detect the
over-all sequence of epigamic displays that result in synchronization
of the physiological states of the sexes throughout the period of
courtship. Possibly all displays, except the post-copulatory one,
occur in no particular order in the courtship period. However, each
ritualized display seemingly strengthens the pair-bond.

Swaying has been recorded in a variety of situations of a sexual and
semi-sexual nature for the Solitary Vireo (_V. solitarius_; Townsend,
1920:158) and the Red-eyed Vireo (Tyler, 1912:230; Bent, 1950:342). In
every instance the body feathers of the swaying birds were sleeked.
Courtship behavior in any species of North American vireo seems
closely to resemble that of any other; pairing and nestbuilding of a
female _V. solitarius_ and a male _V. flavifrons_ as reported by
Hauser (1959:383) support the idea of close resemblance.

A marked similarity will be detected between certain basic elements of
aggressive and epigamic displays. These basic elements are wing- and
tail-flicking, tail-fanning, and high-intensity delivery of the
_chee_. Pouncing and supplanting attacks are essentially similar. Such
similarities suggest either a common origin for certain aggressive and
epigamic displays or the derivation of one from the other.

High-intensity _cheeing_ is obviously a function of excitement,
whether in conjunction with hostility or sexual behavior. According to
Andrew (1956:179), flicking of wing and tail in passerines are
intention movements of flight. These actions have been emancipated
from incomplete take-offs and incorporated in ritualized courtship and
agonistic behavior. In incipient courtship behavior the male is
governed by three conflicting tendencies; to flee, to attack, or to
behave sexually before his mate (Tinbergen and Hinde, 1958:256). When
pairing, Bell Vireos interrupt sexual chase with "greeting
ceremonies," the male's tendency to attack and the female's tendency
to flee are momentarily reduced, and the forming bond is strengthened.
Thus, the intention movements become an integral part of courtship.

In situations where attacking and fleeing are the two conflicting
tendencies, wing-flicking and tail-flicking are incorporated into
threat display, but do not lose all of their original function, for
they facilitate attack. Tail-fanning, as a display element, increases
the awesome aspect of the threatening bird and in courtship presumably
makes the sexes more attractive to one another.

Courtship feeding has not been recorded for the Bell Vireo. In
general, it is unknown in North American vireos, with the exception of
the red-eye (Lawrence, 1953:53). It would serve no "practical" purpose
in the Bell Vireo since the male regularly relieves the female during
incubation, thus allowing her ample opportunity to forage. In the
Red-eyed Vireo, only the female regularly incubates, and courtship
feeding is definitely functional. Nolan (1960:228) described a brief
pecking or pulling with their bills between pairing birds. This may be
incipient "symbolic" courtship feeding, or perhaps mutual preening.

SELECTION OF NEST-SITE AND NESTBUILDING

As far as can be determined, the nest-site is selected by the female.
Typically, the pair makes short, low-level flights from tree to tree
with the female invariably in the lead. The birds usually forage
within each tree; the female interrupts this activity to inspect small
forks of low, pendant branches and the male occasionally pauses to
sing. The singing is loud but not particularly regular, as it is later
when the male accompanies the female during actual nestbuilding.
Method of selection of site resembles that described by Lawrence
(1953:53) for the Red-eyed Vireo.

Nests are suspended from lateral or terminal forks about 27 inches
high in bushes and small trees that, in the study area, averaged 11
feet, four inches in height (Table 5). The height above ground of the
nests does not vary appreciably as the season progresses as is the
case with nests of Red-eyed Vireos, for which Lawrence (1953:54) noted
that late nests were placed higher than those built earlier in the
season.

Most nests are so situated that they are protected and concealed by
the dense foliage of trees. Where nests are placed in low bushes, as
coralberry or dogwood, the bush is invariably overhung by the foliage
of a much taller shrub or tree.

The nest tree or shrub was in every instance situated at the edge of a
thicket or isolated from adjacent trees by several feet. Preference
for open situations is characteristic of the species. In contrast, the
nest of the White-eyed Vireo (Bent, 1950:229) is placed toward the
center of thickets.

In the choice of sites in the study area, the Bell Vireos were almost
unopposed by other avian species, owing to the size of the fork
utilized and the fact that the nests are located peripherally, rather
than centrally, in the bush or tree. This lack of competition for a
nest-site provides a Bell Vireo with an ample supply of nest-sites
within any one territory.

TABLE 5. NEST-SITES UTILIZED IN 1960.

Selection of the first nest-site may take as long as two days,
possibly owing to incomplete development of the nesting tendency, but
more likely to a general lack of familiarity with the territory.
Red-eyed Vireos require five to six days to choose the first nest-site
(Lawrence, 1953:54). Later sites of the Bell Vireo are chosen in as
little as three hours. Nest 1-c (1960) was abandoned at about 11:00
a.m. on May 14, 1960, when part of the thicket on the edge of which
this nest was located was removed by brush-cutters clearing a power
line right-of-way. By 2:00 p.m. this pair had begun construction of
1-d (1960) in an Osage orange 110 feet southwest of 1-c (1960).

This particular site is of further interest because it is the same one
utilized for nest 1-a (1960). In all, four instances of utilization of
a nest-site a second time were recorded. Two-a (1960) and 2-d (1960)
were built in the same fork; 1-c (1960) and 1-h (1960) were in the
same tree, but not the same fork. It should be mentioned that 1-a
(1960) and 2-a (1960) were abortive attempts that did not progress
beyond the suspension apparatus. Nice (1929:16) recorded a similar
instance of the re-use of a nest tree, but different forks were used.

Re-use of an exact nest-site would ordinarily be impossible if the
initial attempt were not abortive, because the presence of a completed
nest would pose problems in construction with which the birds would
probably be unable to cope. (A report by Morse in Bent, 1950:256 of a
double nest indicates that this may not always be true. At the time of
discovery one nest contained two eggs and the other nest contained
young.) Since nests are used only once there would be no tendency to
adopt the old nest. However, abortive nests, usually little more than
a few strands of nesting material secured to the fork, might stimulate
the birds to continue building. Re-use of a single nest-site in 15.8
per cent of 38 nests built in 1960 seems to be more than fortuitous
circumstance. This re-use may have physiological benefits in
conjunction with apportionment of energy for other nesting activities,
because rapid location of a nest-site would mean that energy normally
expended in searching and selecting could be rechanneled for actual
construction. In each of the instances of rebuilding, the new nest
was begun on the same day that the previous nest was abandoned.

The re-nesting of pair 9 (1960) is worthy of note. These birds were
established in the elm thicket on Clark land. Elm was by far the most
abundant tree, with dogwood, Osage orange and honey locust also
relatively common. There were only six boxelders in the territory and
yet the four nests built by this pair were placed in them. This is the
only instance of seeming preference.

_Building_

Nestbuilding by Bell Vireos can be best discussed in terms of the
phases of construction described for the Red-eyed Vireo, Lawrence
(1953:57), which are: (1) construction of the suspension apparatus,
(2) construction of the bag, (3) lining of the bag and smoothing and
polishing of the exterior, and (4) adornment of the exterior. Red-eyes
(Lawrence, 1953:59) may continue adornment far into the period of
incubation. Both the male and female Bell Vireo have been observed to
add spider egg sacs and other silk to the exterior of the nest as late
as the sixth day of incubation.

Nice (1929:16) recorded only the female Bell Vireo building, but she
did recall, from previous studies, having seen males aiding somewhat.
Pitelka and Koestner (1942:102) wrongly concluded that the female Bell
Vireo builds unaided, but Hensley (1950:243) observed that both sexes
participated in nestbuilding, and Mumford (1952:229) reported two
instances of building by both adults. His description of the
activities viewed in mid-May suggest that they were of the
transitional period between the first and second phases. On the second
occasion he recorded both adults building during the second phase.
Since no details accompany this second observation I assume that it
pertained to activity not necessarily typical of this phase of
construction. Whereas both sexes of the Bell Vireo cooperate in
building the nest, only the female Red-eyed Vireo builds according to
Lawrence (1953:56). But Common (1934:242) saw both Red-eyed Vireos
building a nest.

The suspension apparatus is constructed by only the male on the first
day. He punctuates each trip to the nest with song. The single song
phrase is given from three to eight times when the male, carrying
nesting material in his bill, arrives in the tree. Typically, he
alights on several perches within the nest tree before flying to the
nest. He often interrupts his work with several songs; when he has
finished adding a load of material he sings from several perches
within the nest tree before departing. The male periodically stops
building to court the female.

In eight hours (494 minutes) of observing the first phase of
construction at five different nests, I saw the female come to the
nest 28 times; the male made 95 trips. The female came alone only
once, and brought nesting material ten times, but did not build; on
the other 18 occasions her visits were brief and she usually confined
her activities to an inspection of the nest. Twenty of the visits by
the female were made late in the first phase, marking a gradual
transition to her assumption of building responsibility. (The delay by
the female in beginning to build is puzzling; because all evidence
indicates that she helps select the nest-site, I would expect her to
help with the initial building. There seems to be no clear advantage
in her delay in beginning to build.) The courtship and building
activities of the male plus the presence of a partly completed nest
seem to stimulate the female to commence building. Her visits become
more frequent as construction of the suspension apparatus nears
completion. At a time early in the second day the transition has taken
place, and the female becomes the sole worker.

On May 7, 1960, male 2 (1960), at the time unmated, was observed as he
came upon a nest of the previous year. The nest, after a year's
weathering, suggested in appearance perhaps an early second-day nest.
The bird flew to the nest and tugged and wove loose strands of grass
for three minutes. Before leaving the site, the bird sang twice from
different perches. This observation suggests that a partly constructed
nest can elicit nestbuilding behavior, even in an unmated male.

The techniques of building by the male consist primarily of laying
pieces of grass or bark across the fork, or along one of its branches,
and then fastening them in place with pieces of animal silk. Once a
"racket" has been formed, spider egg cases and plant down are emplaced
among the fibers. The male employs weaving, twisting, and pecking
motions of the head to emplace material.

As previously indicated, the female is the principal worker in the
second and third phases of construction. The male infrequently visits
the nest, but regularly visits the nest tree. The molding of the bag
is accomplished by piling leaves, grasses and plant down onto the
suspension apparatus. This material is also bound in with animal silk.
As the amount of material accumulates, the female begins to trample it
and gradually the bag takes shape. When trampling is first attempted,
the nest often fails to support the female and she falls through the
bottom of the nest. Such an occurrence was observed on May 23, 1960,
on three consecutive trips by female 1 (1960), in constructing nest
1-e (1960). As the bag deepens, additional strands of grass are added
to the wall and woven into place.

The male is extremely attentive during this and the following phase.
He follows the female as she gathers nest-material accompanying both
this activity and her building with rapid song; he may give an average
of seven song phrases per minute. The male brings to the nest a strand
of grass, or some other material, about every twentieth trip. He
frequently inspects the nest and the activities of the female from
perches near the nest. Construction of the bag is ordinarily completed
in the third day.

The third phase, the lining of the interior and the smoothing of the
exterior, involves an additional one and one-half to two days.
Smoothing of the exterior refers to tightening of the grasses woven
into the bag and addition of more animal silk. In lining the nest, the
female stands on one of the branches of the fork and emplaces one end
of a long, thin strand of some relatively stiff piece of grass or
strip of bark. She then jumps into the bag and, while slowly turning
around, pecks it into place, thus coiling the strand neatly around the
interior of the bag.

As previously mentioned, the fourth phase overlaps the periods of
lining, smoothing, egglaying, and incubation. The principal activity
is the addition of white spider egg sacs to the exterior. The trips
are infrequent; but, occasionally, birds will interrupt an hour of
incubation with three or four minutes of active adornment, during
which several trips may be made. Both sexes participate in this phase.

_Gathering of Nesting Material_

Nesting materials were gathered anywhere within the territory.
Occasionally materials were collected from within the nest tree, but
usually they were obtained 20 to 200 feet from the nest-site. On
several occasions I observed birds inspecting stems or branches where
bark was frayed. Loose ends are grasped in the beak and torn free with
an upward jerk of the head. Possibly the notch near the distal end of
the upper mandible aids in grasping these strands. Plant down is first
extracted and then rolled into a ball by means of the beak while held
with the feet before being transported to the nest.

_Length and Hours of Nestbuilding_

As indicated by Nolan (1960:230), accurate determination of the length
of nestbuilding is difficult because of continued adornment and
polishing after the nest is functionally complete. Most of the early
nests for which I have records took from four and one-half to five
days to construct. A four-to five-day period of building is reported
by other observers (Nice, 1929:16; Pitelka and Koestner, 1942:99;
Hensley, 1950:242; Nolan, 1960:230).

One instance of protracted building was recorded. Nest 6-d (1960) was
begun on May 29, 1960, and not completed until nine days later on June
6, 1960. In contrast nest 1-g (1960) begun on May 31, 1960, was
finished three days later on June 2, 1960. Nestbuilding occurs between
the hours of 6:00 a.m. and 5:30 p.m. Heavy rain in the early morning
may delay building.

_Abortive Nestbuilding Efforts_

Eight of 38 nests started in 1960 were never completed (Table 6). Six
of these abortive attempts were abandoned during, or shortly after,
the completion of the suspension apparatus. Five of these nests were
abandoned because the female did not begin building following the end
of work by the male. The early abandonment of the other three nests
1-a (1960), 2-c (1960) and 6-e (1960) was attributable to the
interruption of building by the male because of heavy rain and
protracted territorial conflicts. The occurrence of these abortive
nests at any time within the nesting efforts of a single pair
indicates that such attempts are not examples of "false nestbuilding."

_Renesting_

Renesting after desertion or successful fledging occurs within two to
thirty-six hours. Young were fledged from 1-a (1959) on June 19, 1959,
and nest 1-b (1959) was discovered when late in the second phase of
construction on June 22. If the nest was started on June 20, then
renesting took place within 15 hours after fledging.

_The Nest_

Several authors have described various aspects of the nest of the Bell
Vireo, notably Goss (1891:535); Simmons (_in_ Bent, 1950:256), Nice
(1929:13) and Nolan (1960:230-231). I can add but little to these
descriptions.

The nest itself is a compact structure composed of strips of bark and
strands of grasses that are interwoven and tightly bound with animal
silk. The floor of the cup is first lined with a layer of small leaves
and then the entire interior is lined with fine stems or strips of
bark. Feathers are occasionally used to pad the bottom prior to
lining, as are pieces of wool and milkweed down. Nest 2-e (1960) had
been packed with small pieces of soil bearing moss prior to lining.

TABLE 6. ABORTIVE NESTING ATTEMPTS IN MAY AND JUNE OF 1960.

Early nests tend to be bulkier, having thicker walls and bottoms than
later efforts. However, nests in May were found to have 16 per cent
thicker bottoms and 41 per cent thicker walls than nests in June
(Table 7). Standard nest measurements do not show this to be so, for
the exterior and interior diameters at the rim are governed by the
angle between the two branches of the fork.

TABLE 7. DIMENSIONS OF NESTS IN MAY (1960) AND JUNE (1960).

========================================================
Measurements | May (N 10) | June (N 8)
------------------------+---------------+---------------
External depth | 61.6 mm. | 59.3 mm.
Depth of cup | 45.5 mm. | 46.3 mm.
Outside diameter | 57.3/55.5 mm. | 54.3/53.5 mm.
Inside diameter | 43.4/42.2 mm. | 45.5/43.9 mm.
Thickness of forward | |
wall 1 inch below rim | 13.8 mm. | 7.6 mm.
Thickness of bottom | 11.3 mm. | 4.6 mm.
------------------------+---------------+---------------

EGGLAYING AND INCUBATION

_Egglaying_

Egglaying begins the first or second day after completion of the nest.
The female sits in the nest occasionally for periods of five to
twenty-five minutes on the day the nest is completed. This is
interrupted by periods of nest-adornment and foraging; such activities
sometimes keep the female off the nest for several hours. Prior to the
laying of the first egg, only the female is seen on the nest,
although the male is often seen sitting quietly within the nest tree a
few feet from the female. The infrequency of the "congested" song and
the alarm (_eh-eH-EH_) after the inception of "broodiness" indicates
the waning of courtship behavior. As later in incubation only the
"normal" song and the scold are heard.

Eggs are laid early in the morning prior to 5:30 a. m. according to
Nolan (1960:232). The nest is usually left unoccupied for considerable
periods after the first egg is laid, but, on the first day of laying,
both sexes have been observed sitting for brief periods averaging ten
minutes in length. Eggs are laid at one-day intervals until completion
of the clutch. I found incubation to begin with the second egg.

_Clutch-size_

The average clutch-size of the Bell Vireo in Kansas, based on
thirty-three records, is 3.39 eggs (Table 8). Seasonally, the largest
average clutches are produced in the middle of the breeding season,
that is, in June. Lack (1947:308-309) indicates that in European
passerines the highest seasonal average clutch-sizes likewise occur in
June. The largest average clutch-size in the Bell Vireo is presumably
related to some aspect of the availability of food.

TABLE 8. AVERAGE NUMBERS OF EGGS PER NEST (NUMBER OF RECORDS
IN PARENTHESES)[F].

========================================================
| | | | Mean
Year | May | June | July | annual
| | | | clutch-size
----------+---------+----------+---------+--------------
1959 | 3.0 (7) | 3.2 (12) | 3.0 (1) | 3.06
1960 | 3.3 (6) | 3.83 (5) | 4.0 (2) | 3.72
----------+---------+----------+---------+--------------
1959-1960 | 3.17 | 3.52 | 3.5 | 3.39
----------+---------+----------+---------+--------------

[F] These data have been supplemented from the literature
pertinent to Kansas.

Caution is necessary in determining mean clutch-size in the Bell
Vireo. Eggs occasionally disappear from the nest prior to or during
incubation, without subsequent addition of cowbird eggs. Unfamiliarity
with the history of such a nest on the part of the observer would lead
to an inaccurate determination of clutch-size.

Complete clutches are not replaced with the same regularity as are
nests. I have recorded intervals of six to thirty days between
successive clutches. Successful replacement of clutches is determined
by a number of factors: nest-site, completion of a nest, weather,
predation, and parasitism by the cowbird. The difference between the
number of renesting attempts and the successful replacement of
clutches seems to indicate that different physiological processes are
responsible for these two phenomena and that there is lack of
synchrony between them. The development of the ovarian follicle
requires a specific number of days that is not always coincident with
the building of replacement nests. If, in the Bell Vireo, replacing a
nest were solely a responsibility of the female, instead of involving
the male to a considerable extent, it would seem likely that
replacement of nests and the replacement of clutches would be more
closely coordinated.

_Incubation_

Nice (1954:173) considers the incubation period to be the elapsed time
between the laying of the last egg in a clutch and the hatching of
that egg, when all eggs hatch. My data indicate that, normally,
intensive incubation begins when the second egg is laid and lasts
fourteen days in the Bell Vireo. Nice (1929:99) also considered the
incubation period in this species to be fourteen days but believed it
to commence when the third egg was laid. Pitelka and Koestner
(1942:99) noted that the first and second eggs hatched fourteen days
after laying of the second egg. However, they thought incubation began
with the first egg. This would mean a fifteen-day period for this egg.
All the eggs that Nolan (1960:234) marked hatched in approximately
fourteen days. Eight eggs artificially incubated by Graber (1955:103)
required an average of 15.01 days to hatch. As Van Tyne and Berger
(1959:293) indicate, periods of sitting on the nest, even all night,
do not necessarily mean that incubation has begun, for it has been
demonstrated in several species that birds may sit on an egg without
actually applying heat. My own observations demonstrate that the first
egg may be left unattended for several hours at a time on the day that
it is laid.

_The Roles of the Sexes in Incubation_

Both the male and female sit on the eggs in the daytime. My study of
histological sections of ventral epidermis indicates that the male
does not possess a brood patch; the increased vascularization typical
of the brood patch in females is not evident in males. But, the male
loses most of the down feathers of the ventral apterium. Also,
according to Bailey (1952:128), the male Warbling Vireo that sits on
the eggs lacks a brood patch.

Bailey (1952:128) suggests that male passerines lacking brood patches
that habitually sit on eggs do not heat the eggs. Thus it cannot be
considered true incubation since no increase of temperature in the
eggs is effected by such means. He further notes that it is at night
when eggs are likely to experience a drop in temperature that
embryonic development will be impaired. I have no data directly
pertaining to which sex sits at night, but it is presumably the
female, because she is always seen on the nest early in the morning
and late in the evening.

[Illustration: FIG. 4. Comparison of periods of incubation
by both sexes in cold (54° F.) rainy weather (A) and in warm
(82° F.) sunny weather (B).]

If a highly-vascularized brood patch is essential for true incubation,
then it is surprising that males take regular turns on the nest in
cold, rainy weather. On May 20, 1960, male 3 (1960) sat on the eggs
longer than did the female (fig. 4). The temperature during this hour
and a half of incubation was 54° F. One solution to this problem is
supplied by Skutch (1957:74). He indicates that, "the type of the
incubation is determined largely by innate factors, so that it
persists through fairly wide fluctuations in weather, although it may
break down in extreme conditions." Obviously then, in the example
described above, the weather conditions do not qualify as "extreme."
Sitting by the male is certainly functional to some extent for it
relieves the female to forage; furthermore, the eggs are sheltered
from inclement weather and protected from predators. Nolan (1960:232)
suggests similar reasons for incubating by the male and adds the
"conservation of heat supplied to the eggs by the female."

[Illustration: FIG. 5. Daily participation in incubation as
indicated by the sex of the adult on the nest upon approach of
the observer.]

My data, based on incubation beginning with the second egg, indicate
that the female incubates more often daily than the male (fig. 5). The
male sits on the eggs only occasionally in the morning, but almost as
often as the female in the afternoon. Nolan (1960:233) found that 95.5
per cent of the male's time on the nest and only 40 per cent of the
female's time were attributable to the early hours of the day.
Although I lack data on the critical hours of 5:00 a.m. to 6:59 a.m.,
I have enough observations (20) from 7:00 a.m. to 9:00 a.m. to
indicate that the males sit on the eggs infrequently (3 of 20
instances) in those hours. The discrepancy in the two sets of data,
which may be merely an artifact of sampling techniques, does suggest
two possible alternatives: (1) the male sits on the eggs in the
morning and gives the female, who sits on the eggs throughout the
night, an extended rest and an opportunity to forage; (2) the female
continues to sit throughout the morning, especially during the early
hours of daylight, a time of day when the temperature may still be low
enough to impair development of the embryo.

_Relief of Partners in Incubation_

Relief of partners involves some ceremony. When the female is
incubating, the male sings several times as he approaches the nest
tree; the female responds with several _chees_, but otherwise remains
immobile. The male sings several more times upon alighting in the nest
tree whereupon the female _chees_ again and flies directly from the
nest. A few seconds later the male appears at the edge of the nest
and, after inspecting the eggs, hops in and settles upon them. When
the male is sitting he is notably anxious prior to an exchange with
the female, often arising and craning his neck as he surveys the
surrounding vegetation, seemingly searching for his mate. The singing
of the male and the calling of the female serve as signals,
coordinating the exchange.

NESTLING PERIOD

_Hatching Sequence_

As indicated earlier, hatching normally occurs fourteen days after the
second egg is laid. Hatching of the young was staggered at three nests
under observation. In nest 2-b (1959) the first young hatched on June
8, 1959, the second on June 10. In 3-b (1959) one young hatched each
day from the 12th through the 14th of June. In 5-a (1959) two young
hatched on June 15, the third on June 16, and the fourth on June 17.
Size of the young differed notably for about three days as a result of
staggered hatching, but after that day the younger birds tended to
catch up in size with their older brood-mates. The fourth young in
nest 5-a (1959) grew steadily weaker and was missing from the nest on
June 23, 1959. Staggered hatching is usually thought to be related to
the availability of food that will insure survival of at least some of
the nestlings when a shortage of food exists. It is doubtful that
staggered hatching has adaptive significance in the Bell Vireo, since
there seems to be no shortage of food for the young. In small
passerines such as the Bell Vireo the principal problem is to insure
fledging as quickly as possible because of the danger from predators.

_Development of the Nestlings_

Young are pinkish at hatching and devoid of visible natal down. Du
Bois (_in_ Wetherbee, 1957:380), inspected day-old nestlings by means
of a magnifying glass and was unable to detect any down. Nolan
(1960:236) also indicates that the young are naked at birth and that
the "body color is between flesh and rufous except where folds of the
straw yellow skin obscure the underlying colors." The Hutton Vireo
(_Vireo huttoni_) is essentially naked at birth, save for sparse
hairlike down on the head and back (Wetherbee, 1953:380). The Red-eyed
Vireo, according to Lawrence (1953:67) is naked at birth save for a
sparse covering of greyish natal down, on the head, shoulders, and
back.

In the Bell Vireo the pterylae darken slightly on the second day and
the color becomes more intense daily until the quills of the dorsal
tracts, the wings, and the tail break from their sheaths on the sixth
day. In Red-eyed Vireos the pterylae darken by the end of the first
day and the quills break through the skin on the fifth day, erupting
from the sheaths by the seventh day (Lawrence, 1953:67).

From the first day the young are able to squeak. Poking a young bird
was sufficient to elicit this sound, phonetically a nasal _peek_. The
only other vocalization noted throughout the nestling period was an
abbreviated _chee_.

For the first three days tapping the nest or even movement of it
caused by wind would elicit begging. By the fifth day at nest 2-a
(1959) only vigorous agitation of the branch to which the nest was
attached evoked any response. At this nest on June 16, 1959, one young
begged while the other cowered. Cowering is correlated with opening of
the eyes, as the young bird that begged had its eyes only partly open.
Both young cowered on June 19, 1959. Table 9 summarizes the maturation
of the nestling Bell Vireos.

TABLE 9. MATURATION OF NESTLING BELL VIREOS. THE FIRST DAY
THAT AN ACTIVITY WAS OBSERVED IS SHOWN.

==================================================================
| Day of nestling life
+---+---+---+---+---+---+---+---+---+----+----
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11
--------------------+---+---+---+---+---+---+---+---+---+----+----
| | | | | | | | | | |
Eyes open | | | | x | | | | | | |
Feathers erupt | | | | | x | | | | | |
Sound: Squeak | x | | | | | | | | | |
_Chee_ | | | | x | | | | | | |
Begging | x | | | | | | | | | |
Cowering | | | | | | | | x | | |
Head scratching and | | | | | | | | | | |
Preening | | | | | | | | | x | |
Hopping to rim of | | | | | | | | | | |
nest | | | | | | | | | x | |
Fledging | | | | | | | | | | |x[G]
--------------------+---+---+---+---+---+---+---+---+---+----+----

[G] This is the commonest fledging day.

_Parental Behavior_

No eggshells were found in nests on the days of hatching. Presumably
they had been removed by the parents. Nolan (1960:234) indicates
immediate disposition of the eggshell after hatching. Lawrence
(1953:62) suggests that conspicuous removal of eggshells by the female
Red-eyed Vireo informs the male that the young have hatched.

Both sexes brood and the exchange of partners resembles that described
for the incubation period. Decrease in brooding in the daytime begins
about the sixth day of nestling life. Nolan (1960:235) reports a sharp
decrease in brooding when the oldest nestlings are seven days old.
Brooding decreases notably on the sixth day of nestling life in the
Red-eyed Vireo (Lawrence, 1953:62). Nice (1929:17), Hensley
(1950:244), and Nolan (1960:235) report that the female Bell Vireo
assumes a slightly greater role in brooding than the male.

Apparent sun-shading was noted at nest 3-b (1959) at 2:00 p.m. on
June 17, 1959, on the fifth day of the nestling period. The nest
contained three young. An adult flew to the nest; while standing on
its rim the bird dipped its head into the nest six times, afterward
appeared to be eating a fecal sac, than shifted position to the
unattached portion of the rim, gaped three times, thereupon spread its
wings, and sat motionless 35 minutes. In this attitude it formed an
effective shield sheltering the young from direct sunlight penetrating
the thin foliage of the honey locust in which the nest was situated.
The temperature at this time was 95° F., but the sky was partly
cloudy. By 2:30 p.m. the sky had become overcast and the sun passed
behind a cloud. Although sunlight no longer fell directly upon the
nest, the bird remained in the shielding posture for another five
minutes before flying from its perch. Sun-shading was not observed at
either of the other nests containing young; dense overhead vegetation
protected those nests. Sun-shading has been noted in other species
where the nest was poorly protected from the sun. Lawrence (1953:62)
observed this behavior at two Red-eyed Vireo nests in conifers. The
"sun-shield" posture of the Bell Vireo does not correspond to any of
the sunning postures described by Hauser (1957).

_Feeding of the Nestlings_

Both sexes fed the young, and presumably began shortly after the first
nestling hatched. My data indicate that the female does more feeding
than the male (Table 10); in about eight hours of observation a total
of 67 morsels were brought, 43 by the female and 24 by the male, for
an average of once every 7.6 minutes. Nice (1929:17), however,
observed a male to bring food 53 times as compared to 21 visits by the
female. In five and one-half hours of watching, meals were brought
once every 4.9 minutes. Du Bois (_in_ Bent, 1950:257) recorded seven
trips in an hour and forty minutes, or one every fourteen minutes.

At three nests containing young the adults were sometimes silent and
sometimes vocal on their approach. The female often emitted a subdued
_chee_ which, coupled with the vibration of the nest caused by her
arrival, elicited begging behavior from the young. None of the males
was heard to utter such a call, but I have the impression that they
often did call although I failed to hear the sounds. The males did, on
occasion, sing several songs as they approached, even with food held
in their beaks. Such singing elicited begging from the nestlings. Once
the eyes of the young were open they often began begging when a silent
adult was within two or three feet of the nest; begging behavior
probably is elicited by tactile, auditory or visual stimuli in that
order, or, as the nestling period proceeds, by any combination of
these stimuli.

TABLE 10. FEEDING OF THE NESTLINGS.

====================================================
Day of | Length of | Adult involved
nestling period | observation +---------+---------
| | Male | Female
-----------------+--------------+---------+---------
1 | 30 min. | 3 | 5
2 | 60 min. | 1 | 4
3 | 60 min. | 2 | 5
4 | 30 min. | 1 | 4
7 | 60 min. | 4 | 7
2 | 60 min. | 3 | 3
6 | 60 min. | 3 | 6
7 | 30 min. | 3 | 3
9 | 60 min. | 4 | 6
+--------------+---------+---------
Totals | 510 min. | 24 | 43
-----------------+--------------+---------+---------

Not all trips made by parents resulted in successful feeding of young;
some visits seemed to be purely for inspecting the young. On other
occasions the adults experienced difficulty in transferring food to
the young, and, thus thwarted, would themselves eat the food. Nice
(1929:17) estimated that from five to twelve of a total of
seventy-five meals were eaten by adults.

_Nest Sanitation_

Both parents regularly removed fecal sacs from the nest, eating them
for the first five days and thereafter carrying them off and
presumably dropping them. It is doubtful that fecal sacs were actively
removed in the last two days of nestling life as the bottoms of nests
from which young flew away were invariably covered with excrement.

On several occasions a parent brought food to the nest and then
remained perched on the rim alternately peering into the nest and then
preening. Once bill swiping was observed and another time an adult
male sang once. The adult remained at the nest from twenty seconds to
a full minute.

_Fledging_

Eight young were fledged from the four nests in 1959. The nestling
period lasted from nine to twelve days. Human interference may have
been largely responsible for the fledging of the young at nine days.
Pitelka and Koestner (1942:100) found nestling life to last eleven
days. Nolan (1960:235) reports nestling periods varying from 10.5 to
12 days. The young Red-eyed Vireo is ready to leave the nest at ten
days but often remains an additional day before departing (Lawrence,
1953:68).

The oldest nestling at nest 2-a (1959) hopped out on June 17, 1959,
when I disturbed the parents. On this date the juvenal plumage was
only partly developed and the young bird was incapable of flight. By
the tenth day of nestling life the young in all the nests were
observed to hop to the rim, flutter their wings, hop back into the
nest and also to preen and scratch their heads. The young at fledging
are usually completely feathered, but have notably short tails and
relatively short, stubby wings. According to Ridgeway (1904:205) the
juvenal plumage is much like that of the adult.

_Nest Parasites_

Pitelka and Koestner (1942:103) found that incubating adults and later
the young suffered infestation of the northern fowl mite,
_Ornithonyseus sylviarum_. Nolan (1960:241) reports a heavy
infestation of this mite at four nests. Unidentified mites were noted
at four nests in my study area in 1959. Incubating adults were
observed to peck at their breasts and scapulars from the eleventh
through the fourteenth day of incubation. Serious infestations were
not noted at the nests until the ninth day of nestling life. At this
time the young were observed to scratch their heads and peck at their
breasts, scapulars, and the base of their tails. On the day of
fledging the nests were a seething mass of crawling mites; the mites
also extended well up the branches to which the nests were attached.
Nest 1-a (1959), which was discovered on June 18, 1959, presumably on
the day after fledging, was densely covered with mites. Some mites
were still crawling on this nest on June 20, 1959.

FLEDGLING LIFE

On June 20, 1959 I located one young 80 feet northeast of nest 2-a
(1959), about five hours after it had left the nest. One parent was
observed to feed it once. No young were seen thereafter from this or
any other nest. Extreme agitation on the part of one or both parents
on several occasions shortly thereafter, however, suggested the
proximity of the young. Search in the immediate vicinity on each of
these occasions proved fruitless. Three days after fledging their
young, pair 2 (1959) was primarily occupied with courtship activities.
Pair 1 (1959) was involved in courtship and nestbuilding one and
one-half days after the apparent fledging of their young. Nolan
(1960:238) indicates that the young remain within the territory and
perhaps are fed by the parents up until an age of about 40 days.
Sutton (1949:25) and Lawrence (1953:68) present contradictory reports
on fledgling-parent relationships in the Red-eyed Vireo. Sutton
concluded that the young quickly took leave of their parents whereas
Lawrence reported a young bird being fed 35 days after fledging.

_Second Broods_

The curve based on 66 nesting records of the Bell Vireo representing
the breeding activity in northeastern Kansas demonstrates a tendency
toward double-broodedness (fig. 6). The peak of the breeding season is
from May 20 to June 20. The large number (20) of replacement nests
built in late May of 1960 tends to distort the curve of the breeding
data; a second peak about 35 days after the first is evident.

I am of the opinion that the vast majority of vireos are
single-brooded solely by virtue of the limited success of early
nesting efforts, and that in "good" years most pairs would be
double-brooded. Each of the four pairs that successfully raised one
brood in my study area in 1959 renested within a day or two after the
fledging of the young. I do not know the fate of these nests. Nolan
(1960:237) reports at least one instance of a second brood in the
course of his study. Nolan (_op. cit._) notes that the literature, in
general, indicates that vireos are double-brooded, but that his
evidence, mentioned previously, is the only evidence based on banded
birds.

[Illustration: FIG. 6. Breeding season in northeastern Kansas
based on the number of completed clutches in each 10-day
period from May through July.]

REPRODUCTIVE SUCCESS

Only four nests were successful; all of these were observed in 1959.
The principal external factors responsible for nesting failure were
severe weather, predation, parasitism by Brown-headed Cowbirds
(_Molothrus ater_) and human interference (Table 11).

In late winter and early spring of 1960 heavy snow, continuously at a
depth of at least 10 inches, covered most of the Mid-west from
February 20 through March 20. Consequently, the growing season was
some two weeks behind that of 1959. Of all the species in the study
area, the Bell Vireo is the most dependent on dense foliage for cover
and concealment for its nests. Consequently the tardiness of the
season seemingly negatively influenced reproductive success of this
more than any other species of bird in the study area.

_Behavior_

Several aspects of the behavior of the Bell Vireo tend to contribute
to nesting failure. They include:

1. Nest-site. Nests are occasionally suspended from exposed branches.
Occurrences of this sort suggest that the dimensions of the fork are
more important in the choice of a site than availability of cover.

2. Song. The loud, continuous song of the male during nestbuilding
alerts cowbirds and predators to the presence of a nest. The
incongruous habits of the male of singing in the nest tree and while
sitting on the nest may facilitate location by some enemies,
particularly cowbirds.

TABLE 11. EGG MORTALITY IN BELL VIREOS.

======================================================
| | Eggs (N-29)| |
Mortality agents | N[H] | 1959 | N | 1960
| | Per cent | | Per cent
-----------------+------+------------+-----+----------
Predation | 4 | 13.8 | 5 | 10
Weather | 2 | 6.9 | 8 | 16
Cowbird | 14 | 48.3 | 37 | 74
+------+------------+-----+----------
Totals | 20 | 69[I] | 50 | 100
-----------------+------+------------+-----+----------

[H] Number of eggs out of the total number laid lost to
mortality agents.

[I] In 1959 nine eggs were successful (ultimately gave rise to
fledglings).

I am not fully convinced that song from the nest is simply a "foolish"
habit, since snakes, the principal predators with which this species
has to contend, are deaf. My own field observations and the
circumstances of the innumerable instances recorded in the literature
of male vireos singing from the nest suggest that this is a function
of the proximity of the observer. As mentioned elsewhere, vocal threat
is the initial as well as the primary means by which territory is
maintained. Song from the nest evoked by an enemy also serves to alert
the female to danger.

3. Flushing. The Bell Vireo normally relies upon cryptic behavior to
avoid detection at the nest. Most sitting birds, especially the
females, either flush silently when an enemy is about forty feet from
the nest or remain sitting upon the nest tenaciously, refusing to
flush even when touched or picked up. Some birds flushed at
intermediate distances of from three to fifteen feet. In so doing they
revealed the location of their nests. Since none of these
"intermediate flushers" enjoyed nesting success there is possibly some
correlation between these two factors.

_Predation_

Several complete clutches being incubated disappeared from nests that
were unharmed. Absence of eggshells in the vicinity suggests predation
by snakes.

On May 25, 1960, I found a _Peromyscus_ climbing toward nest 1-a
(1960). The mouse moved to within two inches of the nest whereupon I
removed the mouse. Such small rodents constitute another potential
source of predation.

_Cowbird Parasitism_

In this study the failure of 12 of 35 nests can be directly attributed
to cowbird interference. It is well established that the incidence of
cowbird parasitism of Bell Vireo nests is high (Friedmann, 1929:237;
Bent, 1950:260-261). Nolan (1960:240) found only one nest of eight
studied to be parasitized by cowbirds. He indicates that this is
surprising in view of the heavy molestation of the Prairie Warbler
(_Dendroica discolor_) in the same region. A possible explanation of
this phenomenon seems to lie in the much greater abundance of the
Prairie Warbler in comparison to that of the Bell Vireo. In my study
area the incidence of cowbird parasitism on Bell Vireos in 1959 and
1960 greatly exceeded that of all other nesting species that were
parasitized (Table 12).

As indicated previously, the female Bell Vireo leaves the nest
unoccupied several hours at a time in the transition period between
completion of the nest and the start of egglaying. Such behavior early
in the morning certainly would facilitate deposition of cowbird eggs.
Early in the nesting period the mere presence of a cowbird egg in the
nest prior to the laying of the host's first egg leads to abandonment
of the nest. This seems to be correlated with the relative strength of
the nesting tendency; anyhow cowbird eggs laid in later nests prior to
the appearance of the host's own eggs did not cause the nesting birds
to desert. The Bell Vireo does abandon the nest when all but one of
its own eggs have been removed by the cowbird. Mumford (1952:232)
records the removal of a cowbird egg by the host birds and I recorded
a similar instance involving nest 2-b (1960). On May 14, 1960, I
found one punctured cowbird egg on the ground about 10 feet west of
this nest. Occasionally a cowbird egg is buried beneath the lining of
a nest. Mumford (1952:23) observed this in mid-May in 1951 and I
observed pair 8 (1960) actively covering with building material a
cowbird egg on July 5, 1960. Covering a cowbird egg constitutes
effective removal. Since the egg cannot be turned, an adhesion
develops.

TABLE 12. INCIDENCE OF COWBIRD PARASITISM OF THE BELL VIREO
COMPARED WITH OTHER PASSERINES IN THE STUDY AREA IN 1959 AND
1960.

=========================================================
| Bell | Other
| Vireo |passerines
--------------------------------------+-------+----------
Total nests examined containing | |
at least one host egg | 35 | 43
Total nests parasitized | 24 | 14
Total number of cowbird eggs | 33 | 23
Per cent of nests parasitized | 68.6 | 32.6
Total number of cowbird eggs per nest | .94 | .54
--------------------------------------+-------+----------

The percentage of cowbird eggs hatched in relation to the number laid
is relatively low. For instance, Mumford (1952:231) has only one
record of a young cowbird successfully raised by a Bell Vireo. The
data available in Bent (1950:260-261) also indicate that the
percentage of cowbird eggs hatched is small. The Bell Vireo is less
tolerant of cowbird parasitism than are many of the species so
victimized, but is not so intolerant as the Robin, Catbird, and the
Yellow-breasted Chat (Friedmann, 1929:193).

SUMMARY

1. The behavior of a small population of Bell Vireos was studied in
the spring and summer of 1959 and again in 1960 in Douglas County,
Kansas, and results are compared with previous studies elsewhere.

2. The Bell Vireo sings more often daily and throughout the nesting
season than do the majority of its avian nesting associates. Six types
of vocalizations are readily distinguishable in the field: primary
song, courtship song, distress call, alarm note, specialized male call
note or _zip_, and the generalized call note or _chee_.

3. Territories are established in early May and occupied throughout
the breeding season and post-breeding season. The average size of the
territories in 1960 was 1.25 acres. Shifting of territorial boundaries
occasionally occurs after nesting attempts.

4. Territory is maintained primarily by song, but at least five
aggressive displays are manifest in the early phases of territorial
establishment. These include: (a) vocal threat, (b) head-forward
threat, (c) wing-flicking and sub-maximal tail-fanning, (d) ruffling
and maximum tail-fanning, and (e) supplanting attack.

5. The precise mechanism of pair-formation in the Bell Vireo is not
known. Early courtship activities are characteristically violent
affairs. Absence of sexual dimorphism suggests that behavioral
criteria are used by the birds in sex-recognition; the male is
dominant and the female is subordinate.

6. The principal displays associated with courtship include: greeting
ceremonies, "pouncing," "leap-flutter," pre- and post-copulatory
displays, and the posture, copulation. The marked similarity between
elements of courtship display and aggressive display suggests common
origin or the derivation of one from the other.

7. The nest-site probably is selected by the female. Nests are
suspended from lateral or terminal forks about 2 feet 3 inches high in
small trees and shrubs averaging 11 feet 2 inches in height.

8. Nestbuilding is intimately associated with courtship and is a
responsibility of both sexes. The male builds the suspension apparatus
and the female constructs and lines the bag. Both sexes participate in
adorning the exterior. Construction lasts from four and one-half to
five days.

9. The nest is compact, pendant, and composed of strips of bark and
strands of grasses that are interwoven and tightly bound with animal
silk. Nests built in May are bulkier than those constructed later in
the season.

10. Egglaying begins on the first or second day after the nest is
completed. The eggs are deposited early in the morning. The average
clutch-size of the Bell Vireo in Kansas is 3.39 eggs.

11. Both sexes sit on the eggs, but only the female truly incubates
because the male lacks a brood patch. Incubation lasts fourteen days.

12. The Bell Vireo is double-brooded in "good" years.

13. Nesting failure resulted from severe weather, predation,
parasitism by cowbirds, and human interference. Behavior that
contributes to nesting failure is selection of an unfavorable
nest-site, singing on and near the nest, and the tendency to flush
from the nest in view of potential enemies.

LITERATURE CITED

AMERICAN ORNITHOLOGISTS' UNION

1957. Check-list of North American birds. Fifth ed. Baltimore,
The Lord Baltimore Press, The American Ornithologists'
Union, iv + 691 pp.

ANDREW, R. J.

1956. Intention movements of flight in certain passerines, and
their use in systematics. Behaviour, 10:79-204.

BAILEY, R. E.

1952. The incubation patch of passerine birds. Condor,
54:121-136.

BENNETT, W. W.

1917. Bell's Vireo studies (Vireo bellii Aud.). Proc. Iowa
Acad. Sci., 24:285-293.

BENT, A. C.

1950. Life histories of North American wagtails, shrikes,
vireos and their allies. Smithsonian Inst., U. S. Nat.
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BUNKER, C. D.

1910. Habits of the Black-capt Vireo (Vireo atricapillus).
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CHAPIN, E. A.

1925. Food habits of the vireos; a family of insectivorous
birds. U. S. Dept. Agric. Bull., 1355:1-44.

COMMON, M. A.

1934. Notes on a Red-eyed Vireo's nest. Auk, 51:241-242.

COOKE, W. W.

1909. The migration of vireos. Bird Lore, 11:78-82, 118-120,
165-168.

FITCH, H. S.

1958. Home ranges, territories and seasonal movements of
vertebrates of the Natural History Reservation. Univ.
of Kansas Publ. Mus. of Nat. Hist., 11:3:63-326.

FRIEDMANN, H.

1929. The Cowbirds. Charles C. Thomas, Springfield, Illinois,
xviii + 421 pp.

GOSS, N. S.

1891. History of the birds of Kansas. Topeka, Geo. W. Crane &
Co. Printers and Binders. 692 pp.

GRABER, R. R.

1955. Artificial incubation of some non-galliform eggs. Wilson
Bull., 67:100-109.

HAMILTON, T. H.

1958. Adaptive variation in the genus Vireo. Wilson Bull.,
70:307-346.

HAUSER, D. C.

1957. Some observations on sun-bathing in birds. Wilson Bull.,
69:78-90.

1959. Notes on pairing and nestbuilding of mismatched vireos.
Wilson Bull., 71:383-384.

HENSLEY, MAX

1950. Notes on the breeding behavior of the Bell's Vireo. Auk,
67:243-244.

HINDE, R. A.

1952. The behaviour of the Great Tit (Parus major) and some
other related species. Leiden: E. J. Brill, x + 201 pp.

1956. The biological significance of the territories of birds,
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HINDE, R. A., and TINBERGEN, N.

1958. The comparative study of species-specific behavior. In
Behavior and Evolution (Yale University Press, New Haven),
pp. 251-268.

KLUIJVER, H. N.

1951. The population ecology of the great tit, Parus m. major
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LACK, D.

1947. The significance of clutch-size. Ibis, 89:302-352.

LAWRENCE, L. DE K.

1953. Nesting life and behavior of the Red-eyed Vireo. Can.
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LEWIS, H. F.

1921. A nesting of the Philadelphia Vireo. Auk, 38:26-44,
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LINSDALE, J. M.

1928. Birds of a limited area in eastern Kansas. The Univ. of
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LLOYD, W.

1887. Birds of Tom Green and Concho Counties, Texas. Auk,
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MORRIS, D.

1956. The feather postures of birds and the problem of the
origin of social signs. Behaviour, 9:75-113.

MOYNIHAN, M.

1955. Types of hostile display. Auk, 72:247-259.

MUMFORD, R. E.

1952. Bell's Vireo in Indiana. Wilson Bull., 64:224-233.

NICE, M. M.

1929. The fortunes of a pair of Bell Vireos. Condor, 31:13-20.

1943. Studies in the life history of the song sparrow. II. The
behavior of the song sparrow and other passerines. Trans.
Linn. Soc. N.Y., 6:328 pp.

1954. Problems of incubation periods in North American birds.
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NOLAN, V.

1960. Breeding behavior of the Bell Vireo in southern Indiana.
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PITELKA, F. A.

1959. Numbers, breeding schedule, and territoriality in
Pectoral Sandpipers of northern Alaska. Condor,
61:223-264.

PITELKA, F. A., and KOESTNER, E. J.

1942. Breeding behavior of Bell's Vireo in Illinois. Wilson
Bull., 54:97-106.

RIDGWAY, R.

1889. The ornithology of Illinois. Illinois State Nat. Hist.
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SKUTCH, A. F.

1957. The incubation patterns of birds. Ibis, 99:69-93.

SOUTHERN, W. E.

1958. Nesting of the Red-eyed Vireo in the Douglas Lake
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SUTTON, G. M.

1949. Studies of the nesting birds of the Edwin S. George
Reserve. Part 1. The Vireos. Misc. Pub. Univ. Michigan
Mus. Zool., 74:5-36.

TOWNSEND, C. W.

1920. Supplement to the birds of Essex County, Massachusetts.
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TYLER, W. M.

1912. A vireo courtship. Bird Lore, 14:229-230.

VAN TYNE, J., and BERGER, A. J.

1959. Fundamentals of ornithology. John Wiley & Sons, Inc.,
New York. xi + 624 pp.

WETHERBEE, D. K.

1957. Natal plumages and downy pteryloses of passerine
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113:5:339-436.

_Transmitted November 8, 1961._

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11. A new subspecies of pocket mouse from Kansas. By E.
Raymond Hall. Pp. 587-590. November 15, 1954.

12. Geographic variation in the pocket gopher, Cratogeomys
castanops, in Coahuila, Mexico. By Robert J. Russell and
Rollin H. Baker. Pp. 591-608. March 15, 1955.

13. A new cottontail (Sylvilagus floridanus) from northeastern
Mexico. By Rollin H. Baker. Pp. 609-612. April 8, 1955.

14. Taxonomy and distribution of some American shrews. By
James S. Findley. Pp. 613-618. June 10, 1955.

15. The pigmy woodrat, Neotoma goldmani, its distribution and
systematic position. By Dennis G. Rainey and Rollin H. Baker.
Pp. 619-624. 2 figures in text. June 10, 1955.

Index. Pp. 625-651.

Vol. 8.

Nos. 1-10 and index. Pp. 1-675, 1954-1956.

Vol. 9.

1. Speciation of the wandering shrew. By James S. Findley. Pp.
1-68, 18 figures in text. December 10, 1955.

2. Additional records and extension of ranges of mammals from
Utah. By Stephen D. Durrant, M. Raymond Lee, and Richard M.
Hansen. Pp. 69-80. December 10, 1955.

3. A new long-eared myotis (Myotis evotis) from northeastern
Mexico. By Rollin H. Baker and Howard J. Stains. Pp. 81-84.
December 10, 1955.

4. Subspeciation in the meadow mouse, Microtus pennsylvanicus,
in Wyoming. By Sydney Anderson. Pp. 85-104, 2 figures in text.
May 10, 1956.

5. The condylarth genus Ellipsodon. By Robert W. Wilson. Pp.
105-116, 6 figures in text. May 19, 1956.

6. Additional remains of the multituberculate genus
Eucosmodon. By Robert W. Wilson. Pp. 117-123, 10 figures in
text. May 19, 1956.

7. Mammals of Coahuila, Mexico. By Rollin H. Baker. Pp.
125-335, 75 figures in text. June 15, 1956.

8. Comments on the taxonomic status of Apodemus peninsulae,
with description of a new subspecies from North China. By J.
Knox Jones, Jr. Pp. 337-346, 1 figure in text, 1 table. August
15, 1956.

9. Extensions of known ranges of Mexican bats. By Sydney
Anderson. Pp. 347-351. August 15, 1956.

10. A new bat (Genus Leptonycteris) from Coahuila. By Howard
J. Stains. Pp. 353-356. January 21, 1957.

11. A new species of pocket gopher (Genus Pappogeomys) from
Jalisco, Mexico. By Robert J. Russell. Pp. 357-361. January
21, 1957.

12. Geographic variation in the pocket gopher, Thomomys
bottae, in Colorado. By Phillip M. Youngman. Pp. 363-387, 7
figures in text. February 21, 1958.

13. New bog lemming (genus Synaptomys) from Nebraska. By J.
Knox Jones, Jr. Pp. 385-388. May 12, 1958.

14. Pleistocene bats from San Josecito Cave, Nuevo León,
México. By J. Knox Jones, Jr. Pp. 389-396. December 19, 1958.

15. New subspecies of the rodent Baiomys from Central America.
By Robert L. Packard. Pp. 397-404. December 19, 1958.

16. Mammals of the Grand Mesa, Colorado. By Sydney Anderson.
Pp. 405-414, 1 figure in text, May 20, 1959.

17. Distribution, variation, and relationships of the montane
vole, Microtus montanus. By Sydney Anderson. Pp. 415-511, 12
figures in text, 2 tables. August 1, 1959.

18. Conspecificity of two pocket mice, Perognathus goldmani
and P. artus. By E. Raymond Hall and Marilyn Bailey Ogilvie.
Pp. 513-518, 1 map. January 14, 1960.

19. Records of harvest mice, Reithrodontomys, from Central
America, with description of a new subspecies from Nicaragua.
By Sydney Anderson and J. Knox Jones, Jr. Pp. 519-529. January
14, 1960.

20. Small carnivores from San Josecito Cave (Pleistocene),
Nuevo León, México. By E. Raymond Hall. Pp. 531-538, 1 figure
in text. January 14, 1960.

21. Pleistocene pocket gophers from San Josecito Cave, Nuevo
León, México. By Robert J. Russell. Pp. 539-548, 1 figure in
text. January 14, 1960.

22. Review of the insectivores of Korea. By J. Knox Jones,
Jr., and David H. Johnson. Pp. 549-578. February 23, 1960.

23. Speciation and evolution of the pygmy mice, genus Baiomys.
By Robert L. Packard. Pp. 579-670, 4 plates, 12 figures in
text. June 16, 1960.

Index. Pp. 671-690.

Vol. 10.

1. Studies of birds killed in nocturnal migration. By Harrison
B. Tordoff and Robert M. Mengel. Pp. 1-44, 6 figures in text,
2 tables. September 12, 1956.

2. Comparative breeding behavior of Ammospiza caudacuta and A.
maritima. By Glen E. Woolfenden. Pp. 45-75, 6 plates, 1 figure.
December 20, 1956.

3. The forest habitat of the University of Kansas Natural
History Reservation. By Henry S. Fitch and Ronald R. McGregor.
Pp. 77-127, 2 plates, 7 figures in text, 4 tables. December
31, 1956.

4. Aspects of reproduction and development in the prairie vole
(Microtus ochrogaster). By Henry S. Fitch. Pp. 129-161, 8
figures in text, 4 tables. December 19, 1957.

5. Birds found on the Arctic slope of northern Alaska. By
James W. Bee. Pp. 163-211, plates 9-10, 1 figure in text.
March 12, 1958.

6. The wood rats of Colorado: distribution and ecology. By
Robert B. Finley, Jr. Pp. 213-552, 34 plates, 8 figures in
text, 35 tables. November 7, 1958.

7. Home ranges and movements of the eastern cottontail in
Kansas. By Donald W. Janes. Pp. 553-572, 4 plates, 3 figures
in text. May 4, 1959.

8. Natural history of the salamander, Aneides hardyi. By
Richard F. Johnston and Gerhard A. Schad. Pp. 573-585. October
8, 1959.

9. A new subspecies of lizard, Cnemidophorus sacki, from
Michoacán, México. By William E. Duellman, Pp. 587-598, 2
figures in text. May 2, 1960.

10. A taxonomic study of the middle American snake, Pituophis
deppei. By William E. Duellman. Pp. 599-610, 1 plate, 1 figure
in text. May 2, 1960.

Index. Pp. 611-626.

Vol. 11.

1. The systematic status of the colubrid snake, Leptodeira
discolor Günther. By William E. Duellman. Pp. 1-9, 4 figures.
July 14, 1958.

2. Natural history of the six-lined racerunner, Cnemidophorus
sexlineatus. By Henry S. Fitch. Pp. 11-62, 9 figures, 9
tables. September 19, 1958.

3. Home ranges, territories, and seasonal movements of
vertebrates of the Natural History Reservation. By Henry S.
Fitch. Pp. 63-326, 6 plates, 24 figures in text, 3 tables.
December 12, 1958.

4. A new snake of the genus Geophis from Chihuahua, Mexico. By
John M. Legler. Pp. 327-334, 2 figures in text. January 28,
1959.

5. A new tortoise, genus Gopherus, from north-central Mexico.
By John M. Legler. Pp. 335-343. April 24, 1959.

6. Fishes of Chautauqua, Cowley and Elk counties, Kansas. By
Artie L. Metcalf. Pp. 345-400, 2 plates, 2 figures in text, 10
tables. May 6, 1959.

7. Fishes of the Big Blue river basin, Kansas. By W. L.
Minckley. Pp. 401-442. 2 plates, 4 figures in text, 5 tables.
May 8, 1959.

8. Birds from Coahuila, México. By Emil K. Urban. Pp. 443-516.
August 1, 1959.

9. Description of a new softshell turtle from the southeastern
United States. By Robert G. Webb. Pp. 517-525, 2 plates, 1
figure in text. August 14, 1959.

10. Natural history of the ornate box turtle, Terrapene ornata
ornata Agassiz. By John M. Legler. Pp. 527-669, 16 pls., 29
figures in text. March 7, 1960.

Index Pp. 671-703.

Vol. 12.

1. Functional morphology of three bats: Eumops, Myotis,
Macrotus. By Terry A. Vaughan. Pp. 1-153, 4 plates, 24 figures
in text. July 8, 1959.

2. The ancestry of modern Amphibia: a review of the evidence.
By Theodore H. Eaton, Jr. Pp. 155-180, 10 figures in text.
July 10, 1959.

3. The baculum in microtine rodents. By Sydney Anderson. Pp.
181-216, 49 figures in text. February 19, 1960.

4. A new order of fishlike Amphibia from the Pennsylvanian of
Kansas. By Theodore H. Eaton, Jr., and Peggy Lou Stewart. Pp.
217-240, 12 figures in text. May 2, 1960.

5. Natural history of the bell vireo. By Jon C. Barlow. Pp.
241-296, 6 figures in text. March 7, 1962.

More numbers will appear in volume 12.

Vol. 13.

1. Five natural hybrid combinations in minnows (Cyprinidae).
By Frank B. Cross and W. L. Minckley. Pp. 1-18. June 1, 1960.

2. A distributional study of the amphibians of the Isthmus of
Tehuantepec, México. By William E. Duellman. Pp. 19-72, pls.
1-8, 3 figures in text. August 16, 1960.

3. A new subspecies of the slider turtle (Pseudemys scripta)
from Coahuila, México. By John M. Legler. Pp. 73-84, pls.
9-12, 3 figures in text. August 16, 1960.

4. Autecology of the copperhead. By Henry S. Fitch. Pp.
85-288, pls. 13-20, 26 figures in text. November 30, 1960.

5. Occurrence of the garter snake, Thamnophis sirtalis, in the
Great Plains and Rocky Mountains. By Henry S. Fitch and T.
Paul Maslin. Pp. 289-308, 4 figures in text. February 10,
1961.

6. Fishes of the Wakarusa river in Kansas. By James E. Deacon
and Artie L. Metcalf. Pp. 309-322, 1 figure in text. February
10, 1961.

7. Geographic variation in the North American cyprinid fish.
Hybopsis gracilis. By Leonard J. Olund and Frank B. Cross. Pp.
323-348, pls. 21-24, 2 figures in text. February 10, 1961.

8. Descriptions of two species of frogs, genus Ptychohyla;
studies of American hylid frogs, V. By William E. Duellman.
Pp. 349-357, pl. 25, 2 figures in text. April 27, 1961.

9. Fish populations, following a drought, in the Neosho and
Marais des Cygnes rivers of Kansas. By James Everett Deacon.
Pp. 359-427, pls. 26-30, 3 figs. August 11, 1961.

10. Recent soft-shelled turtles of North America (family
Trionychidae). By Robert G. Webb. Pp. 429-611, pls. 31-54, 24
figures in text. February 16, 1962.

Vol. 14.

1. Neotropical bats from western México. By Sydney Anderson.
Pp. 1-8. October 24, 1960.

2. Geographic variation in the harvest mouse, Reithrodontomys
megalotis, on the central Great Plains and in adjacent
regions. By J. Knox Jones, Jr., and B. Morsaloglu. Pp. 9-27, 1
figure in text. July 24, 1961.

3. Mammals of Mesa Verde National Park, Colorado. By Sydney
Anderson. Pp. 29-67, pls. 1 and 2, 3 figures in text. July 24,
1961.

4. A new subspecies of the black myotis (bat) from eastern
Mexico. By E. Raymond Hall and Ticul Alvarez. Pp. 69-72, 1
figure in text. December 29, 1961.

5. North American yellow bats, "Dasypterus," and a list of the
named kinds of the genus Lasiurus Gray. By E. Raymond Hall and
J. Knox Jones, Jr. Pp. 73-98, 4 figures in text. December 29,
1961.

6. Natural history of the brush mouse (Peromyscus boylii) in
Kansas with description of a new subspecies. By Charles A.
Long. Pp. 99-111, 1 figure in text. December 29, 1961.

7. Taxonomic status of some mice of the Peromyscus boylii
group in eastern Mexico, with description of a new subspecies.
By Ticul Alvarez. Pp. 113-120, 1 figure in text. December 29,
1961.

8. A new subspecies of ground squirrel (Spermophilus
spilosoma) from Tamaulipas, Mexico. By Ticul Alvarez. Pp.
121-124. March 7, 1962.

9. Taxonomic status of the free-tailed bat, Tadarida
yucatanica Miller. By J. Knox Jones, Jr., and Ticul Alvarez.
Pp. 125-133, 1 figure in text. March 7, 1962.

More numbers will appear in volume 14.

Vol. 15.

1. The amphibians and reptiles of Michoacán, México. By
William E. Duellman. Pp. 1-148, pls. 1-6, 11 figures in text.
December 20, 1961.

2. Some reptiles and amphibians from Korea. By Robert G. Webb,
J. Knox Jones, Jr., and George W. Byers. Pp. 149-173. January
31, 1962.

3. A new species of frog (Genus Tomodactylus) from western
México. By Robert G. Webb. Pp. 175-181, 1 figure in text.
March 7, 1962.

More numbers will appear in volume 15.

*** End of this LibraryBlog Digital Book "Natural History of the Bell Vireo, Vireo bellii Audubon" ***

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