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.
====================================================== | | 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 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.
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.