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 pr oximate control of the aggressive response and the ecological conditions se lecting for such systems are poorly understood. In this study, we investiga ted the effects of food amount (food amount hypothesis) and competitive asy mmetry on sibling aggression in black guillemot broods. Parental provisioni ng rates were experimentally manipulated in broods comprising a range of ha tching intervals over a 12-h period. Aggression became evident only after p arental provisioning rates were experimentally reduced. When parental provi sioning resumed, adults did not increase their feeding rate to compensate f or 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 fa vor of the dominant chick. Food-deprived chicks from synchronous broods wer e more aggressive than those from asynchronous broods, suggesting that one benefit of hatching asynchrony in the black guillemot is to establish an ef ficient competitive hierarchy among siblings which minimizes the need for c ostly aggressive interactions. On the following day, sibling aggression cea sed, and chicks regained an equal share of parental feeds. Our results prov ide the first evidence that short-term food shortage per se acts as an init ial trigger for aggression and also show that the aggressive response is co mplicated by factors associated with hatching and laying order.