The amount of food ingested by chicks has often been suggested as being the main proximate factor controlling broodmate aggression in facultatively siblicidal species. Although several experiments have demonstrated that short-term food deprivation causes a temporary increase in aggression, no study has, to our knowledge, experimentally manipulated overall food supplies and considered long-term effects on chick behaviour and life history traits. We provided supplemental food to breeding pairs of black-legged kittiwakes, Rissa tridactyla, over an entire breeding season and compared the aggressive behaviour of their chicks with that of chicks of control pairs. Control A-chicks (first to hatch) showed more frequent and intense aggression than their experimental counterparts. Furthermore, the more A-chicks begged and the lower their growth rate the more aggressive they were. The consequences of increased aggression for B-chicks (second to hatch) were lower begging rate, lower growth rate and lower survival. We thus provide evidence that a sustained increase in food availability affects broodmate aggression and chick survival at the nest and we discuss the various proximate and ultimate causes involved in the evolution of broodmate aggression. ?? 2010 The Association for the Study of Animal Behaviour.