| | 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.

_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