Résumé
Although chicks of waterbirds usually stay in their nesting territory, in a few species they precociously leave the nesting site and join together in a “crèche”. While several authors have suggested a relationship between crèching behavior and reduced aggression in Larids, quantitative data on aggression in crèching species are missing. In this paper, we quantify the level of aggression through the breeding season in two evolutionarily independent species pairs, each consisting of two closely-related crèching and non-crèching species (Black-billed Gull, L. bulleri, crèching; Red-billed Gull, L. scopulinus, non-crèching; versus Slender-billed Gull, L. genei, crèching; and Black-headed Gull, L. ridibundus, non-crèching). Aggressiveness was similar among species until the chicks were 1-2 weeks old and then decreased in crèching species and increased in non-crèching ones. This change in aggressiveness at an age when chicks usually join a crèche strongly suggests it is related to crèching behavior. To broaden our study we suggest a comparative approach including Tern species.