If a highly-vascularized brood patch is essential for true incubation, then it is surprising that males take regular turns on the nest in cold, rainy weather. On May 20, 1960, male 3 (1960) sat on the eggs longer than did the female (fig. 4). The temperature during this hour and a half of incubation was 54° F. One solution to this problem is supplied by Skutch (1957:74). He indicates that, "the type of the incubation is determined largely by innate factors, so that it persists through fairly wide fluctuations in weather, although it may break down in extreme conditions." Obviously then, in the example described above, the weather conditions do not qualify as "extreme." Sitting by the male is certainly functional to some extent for it relieves the female to forage; furthermore, the eggs are sheltered from inclement weather and protected from predators. Nolan (1960:232) suggests similar reasons for incubating by the male and adds the "conservation of heat supplied to the eggs by the female."
Fig. 5. Daily participation in incubation as indicated by the sex of the adult on the nest upon approach of the observer.
My data, based on incubation beginning with the second egg, indicate that the female incubates more often daily than the male (fig. 5). The male sits on the eggs only occasionally in the morning, but almost as often as the female in the afternoon. Nolan (1960:233) found that 95.5 per cent of the male's time on the nest and only 40 per cent of the female's time were attributable to the early hours of the day. Although I lack data on the critical hours of 5:00 a.m. to 6:59 a.m., I have enough observations (20) from 7:00 a.m. to 9:00 a.m. to indicate that the males sit on the eggs infrequently (3 of 20 instances) in those hours. The discrepancy in the two sets of data, which may be merely an artifact of sampling techniques, does suggest two possible alternatives: (1) the male sits on the eggs in the morning and gives the female, who sits on the eggs throughout the night, an extended rest and an 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.