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...