Dispersal Patterns of Western Gulls from Southeast Farallon Island

Abstract

During 1978-1981 I studied age- and sex-related dispersal and foraging patterns of Western Gulls (Larus occidentalis) that breed on Southeast Farallon Island, California. Most adults were sedentary and foraged primarily on oceanic food during all seasons except autumn, when human refuse was of major importance. Autumn was the only period when adults did not maintain breeding territories, and also the time of primary molt when flight capabilities were reduced. During the breeding season many subadults moved north to areas where oceanic productivity is high. During autumn/winter most moved south to the Gulf of the Farallons/San Francisco Bay Area, where birds ate both oceanic food and refuse. Dispersal patterns of first-year gulls varied markedly between years, apparently as a result of annual differences in oceanic productivity. The consistency of movement patterns of other age classes resulted from an age-related increase in fidelity to foraging locations. This was most pronounced in adults. Most adults dispersed each year to the same site, and minimized competition by reducing concentrations at each location. Males were more sedentary than females, probably because they are responsible for securing and holding breeding territories.