The Distribution, Abundance, and Habitat Use of Wintering Iceland Gulls <i>Larus glaucoides thayeri</i> in Northern California and Comparisons with Other <i>Larus</i> Gulls.

Recent studies have provided significant insights, but relatively limited information is available on many aspects of the ecology and behavior of the Arctic-breeding Iceland Gull Larus glaucoides. In this study, the distribution, abundance, and habitat use of wintering Iceland Gulls Larus glaucoides thayeri in Northern California, USA, were analyzed in 30 habitat types over three winters. A total of 6012 individuals was recorded, contributing 2.3% of the total Larus gulls recorded. The mean number of Iceland Gulls recorded per count was highest in sanitary landfills located in the Inner Coast Range, Central Valley, Inner Coast, and San Francisco Bay Plain areas. Mean counts were lower in shoreline and intertidal flat habitats on the Inner and Outer Coasts, and low in other habitats. The relative abundance of Iceland Gulls was similar among the habitats. Habitat use was most similar to the American Herring Gull L. smithsonianus and most different from the Western Gull L. occidentalis. Most Iceland Gulls observed were adults (70%), and the percentage slightly increased from the Outer Coast to more inland areas. The availability of substantial food resources at large landfills in the region, which includes four large metropolitan areas, probably accounts for the relatively large wintering population of Iceland Gulls at the southern end of their winter range. This food source may decrease in the future as new solid-waste management requirements are implemented in California. During the breeding period, primary elements of niche segregation between the Iceland Gull and sympatric large Larus gull species include nesting on coastal cliffs, concentrated use of coastal shoreline and inshore habitats for feeding, and use of different food items. In the non-breeding period, elements include differences in habitat use, feeding ecology, and feeding behavior. Abundant food sources may increase overlap in certain breeding and wintering habitats.