Male and Female Parental Roles in the Western Gull under Different Environmental Conditions

Abstract

I examined variation in parental care in the Western Gull (Larus occidentalis), spending two seasons on Southeast Farallon Island (SEFI), where the population was large and competition for breeding space appeared to be high. During the first season (1973), food appeared to be less abundant than usual. During 1973, male gulls spent more time on their territories than did their mates. In 1974, food appeared to be more abundant; male and female gulls spent similar amounts of time on their territories, but females spent more time in incubation. Chick survival also increased in 1974. A third season was spent on Santa Barbara Island, where food appeared to be less abundant than on SEFI in either year; population size and density were low, however, and there did not appear to be much competition for breeding space. In this colony, females spent considerably more time both on their territories and in incubation than did their mates. Male gulls were found to be significantly larger than female gulls in both populations. Male gulls were also more aggressive than females and performed the bulk of territorial defense. Male gulls fed upon larger food items than did females, brought back heavier loads of food, and fed their chicks more often than did their mates. On SEFI, male gulls were monagamous, and nearly all retained the same mate for three consecutive seasons. These males did engage in some promiscuous activity but were rebuffed by females. This activity was more frequent in 1974. On Santa Barbara Island, males participated in considerable promiscuous activity with unmated females and were observed to copulate with females other than their mates. Received 15 May 1980, accepted 19 February 1981.