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.
Methods
Data collection details are described in the associated publication (Sofaer et al. Journal of Avian Biology). Data are based on video recordings of orange-crowned warbler (Leiothlypis celata) incubation behavior, and observations of incubation period length. The data on nest attentiveness are summarized from the on and off bout data.