Natural History of the Bell Vireo, Vireo bellii Audubon Part 7

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

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

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

I am not fully convinced that song from the nest is simply a "foolish"

habit, since snakes, the princ.i.p.al predators with which this species has to contend, are deaf. My own field observations and the circ.u.mstances of the innumerable instances recorded in the literature of male vireos singing from the nest suggest that this is a function of the proximity of the observer. As mentioned elsewhere, vocal threat is the initial as well as the primary means by which territory is maintained. Song from the nest evoked by an enemy also serves to alert the female to danger.

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

_Predation_

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

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

_Cowbird Parasitism_

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

As indicated previously, the female Bell Vireo leaves the nest unoccupied several hours at a time in the transition period between completion of the nest and the start of egglaying. Such behavior early in the morning certainly would facilitate deposition of cowbird eggs.

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

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

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

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

SUMMARY

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

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

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

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

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

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

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

8. Nestbuilding is intimately a.s.sociated with courtship and is a responsibility of both s.e.xes. The male builds the suspension apparatus and the female constructs and lines the bag. Both s.e.xes partic.i.p.ate in adorning the exterior. Construction lasts from four and one-half to five days.

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

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

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

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

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

LITERATURE CITED

AMERICAN ORNITHOLOGISTS' UNION

1957. Check-list of North American birds. Fifth ed. Baltimore, The Lord Baltimore Press, The American Ornithologists'

Union, iv + 691 pp.

ANDREW, R. J.

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

BAILEY, R. E.

1952. The incubation patch of pa.s.serine birds. Condor, 54:121-136.

BENNETT, W. W.

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

BENT, A. C.

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

Mus. Bull., 197:vii + 411 pp., 48 pls.

BUNKER, C. D.

1910. Habits of the Black-capt Vireo (Vireo atricapillus).

Condor, 12:70-73.

CHAPIN, E. A.

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