Part 10

Eight young were fledged from the four nests in 1959. The nestling period lasted from nine to twelve days. Human interference may have been largely responsible for the fledging of the young at nine days. Pitelka and Koestner (1942:100) found nestling life to last eleven days. Nolan (1960:235) reports nestling periods varying from 10.5 to 12 days. The young Red-eyed Vireo is ready to leave the nest at ten days but often remains an additional day before departing (Lawrence, 1953:68).

The oldest nestling at nest 2-a (1959) hopped out on June 17, 1959, when I disturbed the parents. On this date the juvenal plumage was only partly developed and the young bird was incapable of flight. By the tenth day of nestling life the young in all the nests were observed to hop to the rim, flutter their wings, hop back into the nest and also to preen and scratch their heads. The young at fledging are usually completely feathered, but have notably short tails and relatively short, stubby wings. According to Ridgeway (1904:205) the juvenal plumage is much like that of the adult.

Nest Parasites

Pitelka and Koestner (1942:103) found that incubating adults and later the young suffered infestation of the northern fowl mite, Ornithonyseus sylviarum. Nolan (1960:241) reports a heavy infestation of this mite at four nests. Unidentified mites were noted at four nests in my study area in 1959. Incubating adults were observed to peck at their breasts and scapulars from the eleventh through the fourteenth day of incubation. Serious infestations were not noted at the nests until the ninth day of nestling life. At this time the young were observed to scratch their heads and peck at their breasts, scapulars, and the base of their tails. On the day of fledging the nests were a seething mass of crawling mites; the mites also extended well up the branches to which the nests were attached. Nest 1-a (1959), which was discovered on June 18, 1959, presumably on the day after fledging, was densely covered with mites. Some mites were still crawling on this nest on June 20, 1959.

FLEDGLING LIFE

On June 20, 1959 I located one young 80 feet northeast of nest 2-a (1959), about five hours after it had left the nest. One parent was observed to feed it once. No young were seen thereafter from this or any other nest. Extreme agitation on the part of one or both parents on several occasions shortly thereafter, however, suggested the proximity of the young. Search in the immediate vicinity on each of these occasions proved fruitless. Three days after fledging their young, pair 2 (1959) was primarily occupied with courtship activities. Pair 1 (1959) was involved in courtship and nestbuilding one and one-half days after the apparent fledging of their young. Nolan (1960:238) indicates that the young remain within the territory and perhaps are fed by the parents up until an age of about 40 days. Sutton (1949:25) and Lawrence (1953:68) present contradictory reports on fledgling-parent relationships in the Red-eyed Vireo. Sutton concluded that the young quickly took leave of their parents whereas Lawrence reported a young bird being fed 35 days after fledging.

Second Broods

The curve based on 66 nesting records of the Bell Vireo representing the breeding activity in northeastern Kansas demonstrates a tendency toward double-broodedness (fig. 6). The peak of the breeding season is from May 20 to June 20. The large number (20) of replacement nests built in late May of 1960 tends to distort the curve of the breeding data; a second peak about 35 days after the first is evident.

I am of the opinion that the vast majority of vireos are single-brooded solely by virtue of the limited success of early nesting efforts, and that in "good" years most pairs would be double-brooded. Each of the four pairs that successfully raised one brood in my study area in 1959 renested within a day or two after the fledging of the young. I do not know the fate of these nests. Nolan (1960:237) reports at least one instance of a second brood in the course of his study. Nolan (op. cit.) notes that the literature, in general, indicates that vireos are double-brooded, but that his evidence, mentioned previously, is the only evidence based on banded birds.

REPRODUCTIVE SUCCESS

Only four nests were successful; all of these were observed in 1959. The principal external factors responsible for nesting failure were severe weather, predation, parasitism by Brown-headed Cowbirds (Molothrus ater) and human interference (Table 11).

In late winter and early spring of 1960 heavy snow, continuously at a depth of at least 10 inches, covered most of the Mid-west from February 20 through March 20. Consequently, the growing season was some two weeks behind that of 1959. Of all the species in the study area, the Bell Vireo is the most dependent on dense foliage for cover and concealment for its nests. Consequently the tardiness of the season seemingly negatively influenced reproductive success of this more than any other species of bird in the study area.

Behavior

Several aspects of the behavior of the Bell Vireo tend to contribute to nesting failure. They include:

1. Nest-site. Nests are occasionally suspended from exposed branches. Occurrences of this sort suggest that the dimensions of the fork are more important in the choice of a site than availability of cover.

2. Song. The loud, continuous song of the male during nestbuilding alerts cowbirds and predators to the presence of a nest. The incongruous habits of the male of singing in the nest tree and while sitting on the nest may facilitate location by some enemies, particularly cowbirds.

TABLE 11. EGG MORTALITY IN BELL VIREOS.

====================================================== | | Eggs (N-29)| | Mortality agents | N | 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 | 50 | 100 -----------------+------+------------+-----+----------

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

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 principal predators with which this species has to contend, are deaf. My own field observations and the circumstances 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 eggshells 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 constitute 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 constitutes effective removal. Since the egg cannot be turned, an adhesion develops.