OOS 32-10
California Gull population growth and response to tidal marsh restoration in San Francisco Bay
The California Gull (Larus californicus) population in the San Francisco Bay has increased dramatically over the past three decades. In this urban ecosystem, gulls frequently forage at local landfills and take advantage of other anthropogenic food resources. Gulls are also predators of smaller and less abundant nesting waterbirds. California Gull population growth has been identified as one of the most pressing concerns for management of the San Francisco Bay estuary. Historically, California Gulls nested primarily on levees and dry salt ponds, but salt pond habitat is being converted to tidal marsh as part of the South Bay Salt Pond Restoration Project. As tidal marsh restoration progresses on over 15,000 acres in the Bay, it is unclear how California Gulls will redistribute and how other waterbird populations will respond to the new pressures of this rapidly growing predator population. Information on what drives gull population growth and how gulls respond to landscape changes will help predict the ecological impact of future tidal marsh restoration activities in San Francisco Bay. Each Spring, the San Francisco Bay Bird Observatory (SFBBO) leads a California Gull nesting survey to collect data on 10 active colonies representing all known breeding locations in the Bay Area. From 2008-2010, over a thousand gulls were banded to track movement and colony re-distribution.
Results/Conclusions
We estimated 53,458 California Gulls breeding in San Francisco Bay in 2013. Over the past 30 years, SFBBO’s Colonial Waterbird Program has documented this increase beginning with fewer than 20 gulls in 1980. This increase may be due to the gulls’ use of landfills and other anthropogenic food resources in a highly urbanized landscape. From 2011 to 2012, the Bay-wide California Gull population increased by nearly 40%. In 2010, restoration of a former salt pond for the South Bay Salt Pond Restoration Project displaced the largest gull colony of over 23,000. Displaced birds were re-sighted all over the west coast from 2011-2013, but the majority of sightings were at a nearby salt pond location. We have documented that California Gulls consume the eggs and chicks of other waterbird species, including the federally-threatened Western Snowy Plover. Hazing efforts and abatement have proven successful at keeping gulls from foraging at local landfills and initiating nests in areas used by other waterbird species. These results have implications for other estuarine restoration projects and urban ecosystems where growing gull populations are affecting waterbird communities.