The importance of nighttime length to latitudinal variation in avian incubation attentiveness

Avian incubation provides an opportunity to test how parental behavior and
ecological conditions interact to shape variation in offspring traits
along geographic gradients. In particular, the duration of the incubation
period is shorter at higher latitudes, but the degree to which this
pattern arises from genetic divergence in rates of growth and development
versus from parentally-mediated variation in egg temperatures is
controversial. At higher latitudes parents have higher daytime incubation
attentiveness, i.e., they spend a greater proportion of the day on the
nest. However, interpreting latitudinal variation in behavior is
complicated by latitudinal patterns in ambient temperature and day length.
Here, we use 24-hour video recordings to compare the incubation behavior
of orange-crowned warblers (Leiothlypis celata) in California and Alaska
and test how attentiveness varies between populations and as a function of
temperature. Birds in Alaska had higher nest attentiveness during the day,
despite experiencing similar ambient temperatures. However, when analyzed
over 24-hours, the longer nighttime period in California almost entirely
canceled out daytime attentiveness differences between populations, and
differences in 24-hour attentiveness were small. Our work highlights how
incorporating nighttime incubation behavior qualitatively alters
latitudinal patterns of attentiveness, and how lower ambient temperatures
cannot account for the higher attentiveness in a high latitude population.
These populations differ in their incubation period lengths, and
differentiating between evolved versus environmentally-induced variation
in offspring growth and development will help understand the fitness
consequences of variation in developmental periods.