Removal of a nest-mate elicits an age-dependent increase in plasma corticosterone of nestling Black-legged Kittiwakes

Plasma corticosterone concentrations in birds often increase about 3 min after exposure to a stressor such as capture and handling. When measuring adrenal responsiveness of nestlings in broods with more than one nestling, standardizing capture protocols to equalize the stressor among nestlings if they simultaneously perceive the presence and activity of a researcher as a stressor is logistically difficult. The objective of our study was to determine if nestling Black-legged Kittiwakes (Rissa tridactyla) in broods of two mount a corticosterone response when the first nestling is removed from the nest or, alternatively, if each initiates a corticosterone response only at the time it is handled. We obtained blood samples from one nestling within 3 min of initial disturbance of the nest, and then removed and sampled its sibling 10 min later. For younger nestlings, we found no difference in corticosterone levels between those sampled at 3 min and their sibling sampled at 10 min. In contrast, older nestlings sampled at 10 min after initial nest disturbance had elevated corticosterone levels compared to those sampled within 3 min. In addition, nestlings sampled within 3 min of capture had elevated corticosterone when exposed to protracted periods of investigator disturbance at nearby nests. Our results suggest that it is necessary to treat initial disturbance of the nest as the onset of the stress response for all nestlings in multi-nestling broods when handling older nestlings or nestlings of unknown age. In addition, for species that nest in dense colonies, the presence of an investigator at one nest may be a stressor for nestlings in adjacent nests.