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Intrapopulation variations in diet and feeding behaviors have been found in several species of gulls (HARRIS 1965, EVANS 1975, MONAGHAN 1980, PIEROTTI 1981, NIERUHR 1983, SIBLY & MCCLEERY 1983, MCCLEERY & SIBLY 1986, GREW et al. 1983, PIEROTTI & ANNET 1987). Often the variations in the diet have been attributed to sexual dimorphism. Males are larger than females in gulls (HARRIS & JONES 1969, INGOLFSSON 1969). HARRIS & JONES (1969) suggested that large bills are more suitable than small ones for some feeding activities (e.g. predation) in Herring Gulls Larus argentatus. They hypothesized that sexual dimorphism has been evolved to decrease food competition between mates. However, INGOLFSSON (1969) found no sex differences in stomach contents of gulls collected in Iceland. If the sexual dimorphism has been evolved to decrease food competition between mates, diet variation between sexes is expected to be similar to or larger than that within sexes since body size variation between sexes is larger than that within sexes (HARRIS & JONES 1969, COULSON et al. 1983). However, comparisons of the diet variations between and within sexes have not been made. In the above studies, individual variation in the diet was not measured. Although several studies (HARRIS 1965, MCCLEERY & SIBLY 1986) reported individual variation in the diet among gulls, their sample sizes were small and factors affecting the individual variations have not been clear. In order (1) to compare the diet variations between and within sexes and (2) to better understand the possible factors responsible for these diet variations among gulls, a study was made on Slaty-backed Gulls L. schistisagus on Terui Island, Hokkaido. The Slaty-backed Gull is an appropriate species to study variations in the diet because it is a generalist, i.e. it forages many food types such as fish, marine invertebrates, garbage and seabirds in Hokkaido during its breeding season (WATANUKI 1988).

Sex and Individual Variations in the Diet of Slaty-backed Gulls Breeding on Teuri Island, Hokkaido