to
Nolan (1960:232). The nest is usually left unoccupied for considerable
periods after the first egg is laid, but, on the first day of laying,
both sexes have been observed sitting for brief periods averaging ten
minutes in length. Eggs are laid at one-day intervals until completion
of the clutch. I found incubation to begin with the second egg.

_Clutch-size_

The average clutch-size of the Bell Vireo in Kansas, based on
thirty-three records, is 3.39 eggs (Table 8). Seasonally, the largest
average clutches are produced in the middle of the breeding season,
that is, in June. Lack (1947:308-309) indicates that in European
passerines the highest seasonal average clutch-sizes likewise occur in
June. The largest average clutch-size in the Bell Vireo is presumably
related to some aspect of the availability of food.

TABLE 8. AVERAGE NUMBERS OF EGGS PER NEST (NUMBER OF RECORDS
IN PARENTHESES)[F].

========================================================
| | | | Mean
Year | May | June | July | annual
| | | | clutch-size
----------+---------+----------+---------+--------------
1959 | 3.0 (7) | 3.2 (12) | 3.0 (1) | 3.06
1960 | 3.3 (6) | 3.83 (5) | 4.0 (2) | 3.72
----------+---------+----------+---------+--------------
1959-1960 | 3.17 | 3.52 | 3.5 | 3.39
----------+---------+----------+---------+--------------

[F] These data have been supplemented from the literature
pertinent to Kansas.

Caution is necessary in determining mean clutch-size in the Bell
Vireo. Eggs occasionally disappear from the nest prior to or during
incubation, without subsequent addition of cowbird eggs. Unfamiliarity
with the history of such a nest on the part of the observer would lead
to an inaccurate determination of clutch-size.

Complete clutches are not replaced with the same regularity as are
nests. I have recorded intervals of six to thirty days between
successive clutches. Successful replacement of clutches is determined
by a number of factors: nest-site, completion of a nest, weather,
predation, and parasitism by the cowbird. The difference between the
number of renesting attempts and the successful replacement of
clutches seems to indicate that different physiological processes are
responsible for these two phenomena and that there is lack of
synchrony between them. The development of