embryonic development will be impaired. I have no data directly
pertaining to which sex sits at night, but it is presumably the
female, because she is always seen on the nest early in the morning
and late in the evening.

[Illustration: FIG. 4. Comparison of periods of incubation
by both sexes in cold (54 deg. F.) rainy weather (A) and in warm
(82 deg. F.) sunny weather (B).]

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 deg. 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."

[Illustration: 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