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PAGE 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 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 . 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 , Nice , Du Bois , Pitelka and Koestner , Hensley and Mumford . Bent has summarized the information available on the species through 1943. Nolan 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 is best documented . 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 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 , honey locust , and American elm . 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 forms the understory just below the crest of the hill. Isolated clumps of dogwood and hawthorn 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 , but includes scattered clumps of dogwood, Osage orange, and honey locust. Poplars are the only large trees in this area.

The open areas between the thickets are grown up in red top , bluestem , Switchgrass , Kentucky bluegrass , bush clover and mullen . 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.

Nolan , 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 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" and the White-eyed Vireo occupies "brush thickets, scrubby woodland and woodland edge" . Along the Missouri River in extreme northeastern Kansas, Linsdale 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 .

In more central portions of the area of sympatry, nevertheless, the two species do occur within the same habitat and occasionally within the same thicket ; their morphological and behavioral differences, although slight, probably minimize interspecific conflict. The Bell Vireo and the Black-capped Vireo have been found nesting in the same tree in Oklahoma by Bunker ; 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 to be various wild plums . The widespread distribution and abundance of the exotic Osage orange has greatly augmented the supply of trees suitable for nesting.

SEASONAL MOVEMENT

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 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 , 4 , and 5 . 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 on June 18, 1960.

The average date of departure for 21 years in northeastern Kansas is September 3 . The earliest date is August 14 from Concordia, Cloud County, Kansas . The latest date is September 27 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

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.

The Bell Vireo has been characterized as a thicket forager , 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 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.

Chapin 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 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 , and Southern 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 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 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.

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 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 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 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 , 2 , 4 , and 6 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 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.

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.

The Bell Vireo does not have a distinctive flight song; in fact, it rarely sings or calls while in flight. Nolan 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.

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 san

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