California Gull population growth and ecological impacts in the San Francisco Bay estuary, 1980–2016

The breeding population of California Gulls (Larus californicus) in the San Francisco
Bay estuary increased from 24 individuals in 1980 to a peak of over 53,000 in 2014, then
declined to 38,040 in 2016. The expansion of the breeding population may be related to
the availability of suitable nesting sites in close proximity to anthropogenic food subsidies
at landfills. Telemetry data indicate that California Gull movements are largely dictated by
the two primary landfills in South San Francisco Bay. The large population of California
Gulls has had negative effects on locally breeding shorebirds and terns, especially the Forster’s
Tern (Sterna forsteri), American Avocet (Recurvirostra americana), and Western Snowy Plover
(Charadrius nivosus nivosus). In South San Francisco Bay, California Gulls were responsible
for 13% and 38% of egg predation events at nests of American Avocets and Snowy Plovers,
respectively, and 55% and 54% of chick predation events of American Avocets and Forster’s
Terns, respectively. The forced relocation of the largest gull colony (~24,000) at Pond A6 in
2010 resulted in increased survival of Forster’s Tern chicks at the adjacent colony at Pond A7 in
2011. The California Gull population and its effects on locally breeding shorebirds and terns
are among the most pressing concerns for wetland managers within the San Francisco Bay
estuary, especially for the South Bay Salt Pond Restoration Project. Further research is needed
to evaluate the gull’s reproductive rates, habitat use, and annual movements and so to clarify
its demographics and to quantify its effects on other breeding birds.