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.

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

=========================================================
| Bell | Other
| Vireo |passerines
--------------------------------------+-------+----------
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