eWestern BirdsThe Quarterly Journal of Western Field Ornithologists
Vol. 31, No. 3
Western Field Ornithologists
Nesting Populations of California and Ring-billed Gulls in California
W. David Shuford, Thomas P. Ryan
ABSTRACT: Statewide surveys from 1994 to 1997 revealed 33,125 to 39,678 breeding pairs of California Gulls and at least 9611 to 12,660 pairs of Ring-billed Gulls in California. Gulls nested at 12 inland sites and in San Francisco Bay. The Mono Lake colony was by far the largest of the California Gull, holding 70% to 80% of the state population, followed by San Francisco Bay with 11% to 14%. Butte Valley Wildlife Area, Clear Lake National Wildlife Refuge, and Honey Lake Wildlife Area were the only other sites that held over 1000 pairs of California Gulls. In most years, Butte Valley, Clear Lake, Big Sage Reservoir, and Honey Lake together held over 98% of the state’s breeding Ring-billed Gulls; Goose Lake held 9% in 1997. Much of the historical record of gull colonies consists of estimates too rough for assessment of population trends. Nevertheless, California Gulls, at least, have increased substantially in recent decades, driven largely by trends at Mono Lake and San Francisco Bay (first colonized in 1980). Irregular occupancy of some locations reflects the changing suitability of nesting sites with fluctuating water levels. In 1994, low water at six sites allowed coyotes access to nesting colonies, and resulting predation appeared to reduce nesting success greatly at three sites. Nesting islands secure from predators and human disturbance are nesting gulls’ greatest need.