the ovarian follicle
requires a specific number of days that is not always coincident with
the building of replacement nests. If, in the Bell Vireo, replacing a
nest were solely a responsibility of the female, instead of involving
the male to a considerable extent, it would seem likely that
replacement of nests and the replacement of clutches would be more
closely coordinated.

_Incubation_

Nice (1954:173) considers the incubation period to be the elapsed time
between the laying of the last egg in a clutch and the hatching of
that egg, when all eggs hatch. My data indicate that, normally,
intensive incubation begins when the second egg is laid and lasts
fourteen days in the Bell Vireo. Nice (1929:99) also considered the
incubation period in this species to be fourteen days but believed it
to commence when the third egg was laid. Pitelka and Koestner
(1942:99) noted that the first and second eggs hatched fourteen days
after laying of the second egg. However, they thought incubation began
with the first egg. This would mean a fifteen-day period for this egg.
All the eggs that Nolan (1960:234) marked hatched in approximately
fourteen days. Eight eggs artificially incubated by Graber (1955:103)
required an average of 15.01 days to hatch. As Van Tyne and Berger
(1959:293) indicate, periods of sitting on the nest, even all night,
do not necessarily mean that incubation has begun, for it has been
demonstrated in several species that birds may sit on an egg without
actually applying heat. My own observations demonstrate that the first
egg may be left unattended for several hours at a time on the day that
it is laid.

_The Roles of the Sexes in Incubation_

Both the male and female sit on the eggs in the daytime. My study of
histological sections of ventral epidermis indicates that the male
does not possess a brood patch; the increased vascularization typical
of the brood patch in females is not evident in males. But, the male
loses most of the down feathers of the ventral apterium. Also,
according to Bailey (1952:128), the male Warbling Vireo that sits on
the eggs lacks a brood patch.

Bailey (1952:128) suggests that male passerines lacking brood patches
that habitually sit on eggs do not heat the eggs. Thus it cannot be
considered true incubation since no increase of temperature in the
eggs is effected by such means. He further notes that it is at night
when eggs are likely to experience a drop in temperature that