opportunity to forage; (2) the female
continues to sit throughout the morning, especially during the early
hours of daylight, a time of day when the temperature may still be low
enough to impair development of the embryo.

_Relief of Partners in Incubation_

Relief of partners involves some ceremony. When the female is
incubating, the male sings several times as he approaches the nest
tree; the female responds with several _chees_, but otherwise remains
immobile. The male sings several more times upon alighting in the nest
tree whereupon the female _chees_ again and flies directly from the
nest. A few seconds later the male appears at the edge of the nest
and, after inspecting the eggs, hops in and settles upon them. When
the male is sitting he is notably anxious prior to an exchange with
the female, often arising and craning his neck as he surveys the
surrounding vegetation, seemingly searching for his mate. The singing
of the male and the calling of the female serve as signals,
coordinating the exchange.

NESTLING PERIOD

_Hatching Sequence_

As indicated earlier, hatching normally occurs fourteen days after the
second egg is laid. Hatching of the young was staggered at three nests
under observation. In nest 2-b (1959) the first young hatched on June
8, 1959, the second on June 10. In 3-b (1959) one young hatched each
day from the 12th through the 14th of June. In 5-a (1959) two young
hatched on June 15, the third on June 16, and the fourth on June 17.
Size of the young differed notably for about three days as a result of
staggered hatching, but after that day the younger birds tended to
catch up in size with their older brood-mates. The fourth young in
nest 5-a (1959) grew steadily weaker and was missing from the nest on
June 23, 1959. Staggered hatching is usually thought to be related to
the availability of food that will insure survival of at least some of
the nestlings when a shortage of food exists. It is doubtful that
staggered hatching has adaptive significance in the Bell Vireo, since
there seems to be no shortage of food for the young. In small
passerines such as the Bell Vireo the principal problem is to insure
fledging as quickly as possible because of the danger from predators.

_Development of the Nestlings_

Young are pinkish at hatching and devoid of visible natal down. Du
Bois (_in_ Wetherbee, 1957:380), inspected day-old nestlings by means
of a magnifying glass and was unable