Incubating terns modify risk-taking according to diurnal variations in egg camouflage and ambient temperature

Abstract

Studies of risk-taking by breeding birds have frequently addressed the effect of brood value on the decisions taken by incubating birds when predators approach their nests. However, leaving eggs unattended during predator disturbance may expose embryos to other potentially harmful factors, to which parent birds should respond when making decisions about when to leave or return to their nest. In this study, we show that diurnal changes in flushing behaviour of incubating terns from nests during predator approach were affected by egg camouflage, the terns allowing a closer approach to individual nests when the eggs appeared better camouflaged. Return times to the nests were affected by ambient temperature, with the terns shortening such times at high ambient temperatures, thus diminishing the risk of egg overheating. As a whole, our results show that the decisions of the birds on when to leave or return to their nests depended on shifting payoffs, as a consequence of diurnal variations in both the thermal risks incurred by embryos and egg crypsis. Environmental costs of risk-taking, such as those considered here, should be addressed in studies of risk-taking by breeding birds. This type of study may have implications for our knowledge of cognitive processes that affect risk-taking. When a predator approaches a nest, the incubating bird has to decide at which moment to leave the nest, which may be affected by environmental factors that may hamper embryo viability. We studied flushing/returning behaviour of incubating little terns Sternula albifrons in response to disturbance and show that risk-taking was affected by the occurrence of simultaneous environmental threats according to shifting fitness payoffs. When the eggs appeared better camouflaged, which usually occurred around midday, the terns allowed closer approach to individual nests. The terns shortened the return to their nests with increasing ambient temperature, which took place at midday. Thus, our results show that, by adjusting their responses to shifting payoffs, the terns modulated the risks incurred by their offspring, as eggs were less time exposed to direct solar radiation in midday, when the risk of overheating was higher.