Incubation temperature does not explain variation in the embryo development periods in a sample of Neotropical passerine birds

Abstract

Growth of avian embryos depends on heat transferred from incubating parents. Parental behavior, including nest characteristics, can influence the length of the incubation period by varying the temperature of incubated eggs, the length of parental absences from the nest, and the rate of egg cooling during absences. Reduced parental attendance in response to risk of predation at the nest has been proposed as an explanation for the longer incubation periods of many tropical birds compared to temperate species. We incubated the eggs of eight species of tropical passerine birds, with natural incubation periods between 12 and 19 days, at the same constant temperature, thereby removing variation in incubation temperature and the cooling periods that eggs might otherwise experience when parents take recesses from incubation. If differences in egg temperature, reflecting parental behavior, were responsible for variation in incubation periods, incubation under constant conditions should reduce or eliminate this variation. Constant egg warming did not reduce time to hatch in species with long incubation periods. Instead, it prolonged embryo development in species with shorter natural periods by up to 2 days, and had no effect on species with longer natural periods. Thus, the longer incubation periods of some tropical birds appear to reflect intrinsic differences in the embryo development program rather than extrinsic factors related to parental incubation behavior.