The California least tern (Sterna antillarum browni) nests in colonies on the Pacific coast of California and Baja, Mexico on relativey open beaches where vegetation is limited by the tidal scouring . It could formerly be found in great abundance from Moss Landing, Monterey County, California to San Jose del Cabo, southern Baja California, Mexico. It was impacted in the 19th and early 20th century by the millinary trade which collected feathers for women's hats, but not to the degree that many east coast birds were. The Migratory Bird Treaty Act of 1916 ended the threat, but the least tern plummeted again some decades later due to growing development and recreational pressures which destroyed habitat, disturbed birds, and increased predation by introduced and native species. The construction of the Pacific Coast Highway brought all these threats to much of California's coast. By the 1940s, terns were gone from most beaches of Orange and Los Angeles counties and were considered sparse elsewhere. To avoid humans, some tern colonies nest at more inland mudflat and dredge fill sites, which appears to make them more susceptible to predation by foxes, raccoons, cats and dogs.
When placed on the endangered species list in 1970, just 225 nesting tern pairs were recorded in California . Protection of nest beaches from development, degradation and disturbance, and active predator control programs caused the species to steadily increase to 6,561 pairs in California in 2004 . A portion of the increase in the 1970s is attributable to expansion and greater consistency in survey effort, but greatest increases occurred from 1980 to the present. 57% of 2004 nesting pairs occurred in San Diego County, 26% in Los Angeles and Orange counties, 10% in Ventura County, 1% in San Louis Obispo and Santa Barbara counties, and 6% in the San Francisco Bay . The largest 2004 sites were at Camp Pendleton (21%), U.S. Navy lands at San Diego Bay (16%), Los Angeles Harbor, Pier 400 (15%), Point Mugu (8%), Alameda Point (6%), Batiquitos Lagoon Ecological Reserve (6%), Huntington State Beach (5%), and Tijuana Estuary (5%).
The U.S. Fish and Wildlife Service recovery plan was issued in 1980 and revised in 1985 . Its downlisting and delisting criteria incorporate the total population size, number of colonies, geographic distribution, population viability, and management status. To be downlisted to "threatened" status, the tern must have at least 1,200 breeding pairs distributed in at least 15 "secure" management areas. Each of these management areas must have at least one viable colony. The San Francisco Bay, Mission Bay, and San Diego Bay management areas must have at least 3, 5, and 4 viable colonies repectively. To be viable, a colony must have at least 20 breeding pairs, a three year mean reproduction rate of >= 1.0 fledglings/pair, and be fully protected under a long-term management plan. This would require a minimum of 24 viable colonies. To be recovered and delisted, the tern must have at least 1,200 breeding pairs distributed in at least 20 "secure" management areas. Each of these management areas must have at least one viable colony with a mean five year reproduction rate of >= 1.0 fledglings/pair. The San Francisco Bay, Mission Bay, and San Diego Bay management areas must have at least 4, 6, and 6 viable colonies respectively. This would require a minimum of 33 viable colonies.
The total population size of 1,200 breeding pairs for down and delisting was reached in 1988 and all subsequent years. The downlisting criterion of 24 colonies was met in 1996 and all subsequent years. The delisting criterion of 33 colonies was met in 2003 and 2004. However, neither the down nor delisting requiremets for distribution and viability have been met. While human disturbance has been managed with fencing at most nesting areas, protection for native and non-native predators will require permanent management commitments to ensure continuing viability after the species is recovered and delisted .
 USFWS. 1985. Recovery Plan for the California Least Tern, Sterna Antillarum Browni. U.S. Fish and Wildlife Service, Portland, Oregon. 112pg.
 Keane, K. 2005. California least tern monitoring 1969-2004. Spreadsheet provided by Kathy Keene, Keane Biological Consulting, Long Beach, CA on July 25, 2005.
 Keane Biological Consulting. 2004. Breeding Biology of the California Least Tern in the Los Angeles Harbor, 2004 Breeding Season. Prepared for the Port of Los Angeles, Environmental Management Division, under contract with the Port of Los Angeles, Agreement No. 2316.