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.

[Illustration: FIG. 6. Breeding season in northeastern Kansas
based on the number of completed clutches in each 10-day
period from May through July.]

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[H] | 1959 | N | 1960