Effects of short-term hunger and competitive asymmetry on facultative aggression in nestling black guillemots Cepphus grylle

Siblings in a diversity of species are facultatively aggressive, yet the proximate control of the aggressive response and the ecological conditions selecting for such systems are poorly understood. In this study, we investigated the effects of food amount (food amount hypothesis) and competitive asymmetry on sibling aggression in black guillemot broods. Parental provisioning rates were experimentally manipulated in broods comprising a range of hatching intervals over a 12-h period. Aggression became evident only after parental provisioning rates were experimentally reduced. When parental provisioning resumed, adults did not increase their feeding rate to compensate for the induced food deficit, and the result of sibling rivalry was a change in the allocation of parental deliveries from one of equality to one in favor of the dominant chick. Food-deprived chicks from synchronous broods were more aggressive than those from asynchronous broods, suggesting that one benefit of hatching asynchrony in the black guillemot is to establish an efficient competitive hierarchy among siblings which minimizes the need for costly aggressive interactions. On the following day, sibling aggression ceased, and chicks regained an equal share of parental feeds. Our results provide the first evidence that short-term food shortage per se acts as an initial trigger for aggression and also show that the aggressive response is complicated by factors associated with hatching and laying order.