Corticosterone-implanted chicks transmit stress to parents and neighbors in a colonial seabird

In animals living in groups, stress-induced changes in behavior can be a source of social information, and stressed individuals can potentially become stressors for other social partners, with important consequences for social and population dynamics. Here, we studied stress transmission from experimentally stressed chicks to both their parents and neighbors in the yellow-legged gull (<i>Larus michahellis</i>), a seabird that forms large breeding colonies. To do this, we experimentally increased the level of a stress hormone by corticosterone implant in 2 first-hatched chicks of the brood and observed its effects on their parents and both adults and chicks in the neighboring nests. Two days after the implant, corticosterone-implanted chicks showed reduced basal corticosterone levels, probably due to a physiological feedback response. Exogenous corticosterone promoted behavioral changes in the corticosterone-implanted chicks, showing faster responses to a potential predator attack than the placebo-treated chicks. Eight days after implantation, not only the corticosterone-implanted chicks but also the neighboring chicks showed elevated corticosterone levels after a standardized handling stress compared with the placebo-implanted chicks and their neighbors. The parents and neighbor adults of the corticosterone-implanted chicks showed increased mobbing behavior but reduced aggressive and resting behaviors in comparison with the adult gulls living close to the placebo-implanted chicks. Overall, our results suggest that individual physiological stress in a colony may be socially transmitted within families and neighbors, with potential consequences for colony dynamics.