Natural History of the Bell Vireo, Vireo bellii Audubon.

by Jon C. Barlow.

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 a.s.sociated with the maintenance of territory and with courtship. Among the North American Vireonidae, the behavior of the Red-eyed Vireo (_Vireo olivaceus_) is best doc.u.mented (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 ma.n.u.script. 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 a.s.sisted 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. s.e.xes 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 s.e.xes. Histological cross-sections, prepared by A. Wayne Wiens, of the ventral epidermis of both s.e.xes 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 const.i.tuted 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 a.s.sociated 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 a.s.sociation, 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 princ.i.p.al 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...o...b..culatus_) 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.

[Ill.u.s.tration: 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_), Switchgra.s.s (_Panic.u.m virgatum_), Kentucky bluegra.s.s (_Poa pratensis_), bush clover (_Lespedeza capitata_) and mullen (_Verbasc.u.m thapsus_). Shrubby vegetation occupies about 65 per cent of the total area, but in the Clark portion const.i.tutes 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 compet.i.tion 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 a.s.sociated 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 a.s.sociated 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.