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

INTRODUCTION

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

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 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.

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

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.

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

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

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.