Impact of increased nest temperature on incubation behavior and female health in Eurasian Blue Tit

Abstract The increase in temperature associated with climate change may influence reproductive costs in birds, particularly during the incubation period, which is temperature-dependent and crucial for embryo development. Female birds are usually solely responsible for incubation and should face a tradeoff between the time spent incubating and foraging or performing other self-maintenance activities outside the nest. Here we studied the effects of temperature on incubation behavior, female health and offspring fitness-related traits of Blue Tits (Cyanistes caeruleus). To that end, we experimentally increased the temperature inside nest boxes used by Blue Tits by an average of 3.29 °C as compared to control nests. Females in heated nests spent less time incubating with more frequent incubation sessions as compared to females from control nests. Furthermore, fewer females attending heated nests were infected at 3 d of nestling age, and those infected had lower intensity of blood parasite infections and were infected by a lower number of different parasites. In addition, females from control nests with longer incubation periods exhibited higher infection intensities of Haemoproteus and Lankesterella. This could be due to improved immune resource allocation as incubation effort decreased. However, 10 d later, most of the effects on blood parasites disappear, and we found no effect of treatment on females' body mass or breeding success, suggesting that control females achieve similar reproductive success by investing more time incubating, at the cost of their health. These results indicate that variations in nest temperature can have significant short-term effects that might have consequences for fitness.