Hormone profiles of obligate avian brood parasites during the breeding season

The ultimate explanations for avian brood parasitism have been studied intensively as a model system for coevolution, but little is known about the proximate mechanisms, for example hormonal regulation, underlying brood parasitic behaviour. In this study, we explored seasonal hormone profiles in two brood parasitic Cuculus species breeding in the Republic of Korea. As brood parasites have relatively simple breeding stages without incubation and provisioning, we predicted that during the breeding season individuals would exhibit similar levels of testosterone (T) and stress‐induced corticosterone (CORT), hormones that are known to be closely related to the transition of breeding stages. We also assessed how these hormone profiles were associated with traits such as body size and sex. Overall, male cuckoos showed similarly high T levels throughout the breeding season, as predicted, but individual variation became greater as the season progressed. Individual CORT levels tended to decrease as the season progressed, although the decrease was significant only in Common Cuckoos Cuculus canorus. We also found that male Lesser Cuckoos Cuculus poliocephalus showed a much higher level of T than females, as expected, but this sexual difference was not observed in Common Cuckoos. Our results suggest that the seasonal hormone profiles of avian brood parasites are likely to be similar to typical hormone profiles expected for non‐brood parasites during the breeding season. This may suggest that not only the breeding cycle but also other factors such as social interaction may be affected by hormonal changes. Further studies are needed to fully understand the proximate mechanism of avian brood parasitism.