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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
BY
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 deg. F.,
as it did on many days in June, 1960. Nice (1929:17) mentions a
similar decrease in singing when the temperature exceeded 85 deg. 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 deg. to the horizontal. Occasionally, during nest
building, I have observed the body held at angles as severe as 80 deg.
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 deg. 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 deg. 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.