Recent trends in population size and distribution of Ring-billed and California gulls in the western United States

Abstract

The breeding ranges of the Ring-billed Gull (Larus delawarensis) and California Gull (L. californicus) overlap broadly in the interior of western North America, where their populations have been increasing since the early 20th century. This trend has been attributed to the gulls’ exploitation of new nesting sites at reservoirs and expanded opportunities for foraging at landfills and irrigated agricultural lands, but it might also reflect, in part, a rebound following declines from persecution and loss of historic wetland habitats. Here we update the population status of both species with data from broad-scale surveys of colonies conducted in 11 western states from 2009 to 2011 as part of the Western Colonial Waterbird Survey and surveys from a few coastal colonies during that period. We compare these results with data from the West in the late 1970s to early 1980s. We estimate a minimum of 87,995 Ringbilled and 334,824 California gulls were breeding in the 11 western states from 2009 to 2011. Although recent estimates are higher than those from about 30 years prior, differences in study methods, assumptions, and environmental conditions make comparisons between the two periods difficult. It is clear, however, that the ranges of the two species have expanded south or west: slightly for the Ring-billed Gull and modestly for the California Gull, particularly with its colonization of the San Francisco Bay estuary followed by 30 years of exponential growth. Some new southern colonies, however, have been small, ephemeral, or have had poor reproductive success. These gull species are, of course, adapted to the periodic droughts that occur in the West, but there are limits to their adaptability. Thus, their populations may be affected as human manipulation of water systems and changing weather and climatic conditions increase the frequency of desiccation of foraging habitats and of predators’ access to nesting islands. The challenge ahead will be to monitor gulls throughout the West in the context of continuing human alteration of the environment and both short-term fluctuations and long-term changes in climate. Likewise, it will be difficult to manage gull numbers regionally and locally where human food sources support large concentrations of gulls to the detriment of other nesting waterbirds and shorebirds.