Seasonal Movements, Migration, and Range Sizes of Subadult and Adult Bamforth Lake California Gulls

Abstract

--We investigated seasonal migration patterns of three age classes of California Gulls (Larus californicus). Using band recovery data and reported sightings of patagially marked gulls, we constructed location maps for fledglings, one to two-year-old gulls, and breeding-age adult gulls during five time periods: spring migration, breeding season, early and late fall migration, and winter. Using repeated observations, we also produced a location map with directional vector plots. Gulls followed a triangular pattern of movements. At the conclusion of breeding, gulls migrated west and northwest to the Pacific coast, distributing themselves mainly between San Francisco and British Columbia. During winter, gulls moved south along the Pacific coast and by spring were concentrated in southern California and northern Mexico. The range size was largest among fledglings which provided the northernmost and southernmost observations for the population. Few subadult gulls migrated to the breeding colony. During the breeding season, a substantial portion of breeding-aged adults remained on the Pacific coast and throughout the intermountain west but were not observed at other California gull colonies. While fledglings moved directly toward the Pacific coast at the end of the breeding season, many adults lingered near the colony site and at aquatic habitats south of the colony. We suggest that adults remain longer in the area to feed and to rejuvenate before attempting migration. Received 13 November 1997, accepted 2 April 1998.