SEXUAL DIFFERENTIATION OF AGGRESSIVE BEHAVIOR IN THE CHICKS OF THE BLACK-HEADED GULL (<i>LARUS RIDIBUNDUS</i>, LARIDAE, CHARADRIIFORMES) IN THE NATURAL ENVIRONMENT

Sexual differentiation of social behavior in gulls and its mechanisms remain unexplored beyond the breeding season. We described the development of aggressive territorial behavior in males and females of black-headed gulls at the age of 1–4 weeks, modeling the intrusion of a peer to the nesting territory. We also hypothesized that the sex differentiation of aggressive behavior may appear as a consequence of the development of small early differences between siblings in behavioral stress response. To test this assumption, we investigated the relationship between the priority among brood in aggressive contact with the intruder, and the within-broods level of activity of a chick to stress. To measure the behavioral stress response, we used the social isolation of chicks within a small water area with an island in the middle. Longer attempts to swim outside the fenced area were regarded as a more proactive stress response. At the age of up to 3 weeks, there were no sex differences in the aggressiveness, but the stress response of males was more proactive than that of their sisters, and a more proactive chick in the brood was usually the first to attack the intruder. At the fourth week of life, the aggressiveness of males increased significantly, and in broods of two sexes, brothers began to play a leading role in protecting the nesting territory in the absence of parents. They attacked the intruder more often and made aggressive calls more frequently. Thus, we were the first to reveal sexual differentiation of territorial behavior in gull chicks in the pre-fledging period. Our results suggest that under natural conditions, where the intruder does not persist in staying in the other nest, but recedes immediately when counteracted, more proactive males use to attack the intruder earlier than sisters do. As a result, they gain experience of aggressive interactions, and develop higher abilities of aggressive responses to social challenges.