California Least Tern

Sterna antillarum browni

Description

The California least tern, Sterna antillarum browni, is among the smallest members of the tern subfamily Sterninae, averaging 9 in (23 cm) in length and having a 16-in (40-cm) wingspread. It has a black cap and nape, gray wings with black wingtips, orange legs, and a black-tipped orange-yellow bill. Sexes appear similar. Immature birds have darker plumage and a dark bill; their white heads with dark eye stripes are distinctive.

This species has also been known by the scientific name Sterna albifrons browni.

Behavior

The California least tern is migratory, typically arriving in its breeding area during the last week of April and departing again in August. It has been recorded in the breeding range as early as mid-March and as late as mid-November. Some birds form pair bonds before arriving in the nesting areas. Others pair off within the colony almost immediately, and active courtship may be observed within the first few days after arrival.

Courtship follows a well-defined pattern, beginning with a "fish flight" in which a male carrying a fish is joined by one or two other terns in a high-flying aerial display. In later stages of courtship, the male holds a small fish in his beak as he courts the female. During copulation, the female takes the fish from the male and eats it. Nests are small depressions scooped out of the sand or dirt. If the soil is hard, the bird will use a natural depression. After eggs are laid, nests are often lined with shell fragments and small pebbles.

Least tern eggs are buff-colored with brown and purple streaks and speckles; normal clutches consist of two or three eggs. Both parents participate in incubation, with the female taking the greater role. Newly hatched chicks are weak and helpless, but by the second day can make short walks from the nest. Nestlings can fly after about 20 days, but are not proficient feeders until after migration. The California least tern eats small fish, which it catches by diving head first into the water.

Habitat

The least tern usually chooses its nesting location in an open expanse of light-colored sand, dirt, or dried mud beside a lagoon or an estuary. Formerly, sandy ocean beaches were regularly used, but increased human activity has made most beaches uninhabitable. In recent years, terns have nested on mud and sand flats away from the ocean or on man-made landfills. Least terns live in colonies, which are less dense than those of most other terns. The California least tern typically fishes in shallow estuaries and lagoons; colonies occasionally forage in the ocean. Fish known to be eaten, in order of importance, are northern anchovy, topsmelt, various surf-perch, killifish, and mosquitofish.

Distribution

The historic breeding range of this subspecies extended along the Pacific Coast from Moss Landing (Monterey County), California, to San Jose del Cabo (southern Baja California), Mexico. Since 1970, nesting sites have been recorded from San Francisco Bay to Bathia de San Quintin, Baja California. The nesting range in California has apparently always been widely discontinuous, with the majority of birds nesting in southern California from Santa Barbara south through San Diego County. Between the city of Santa Barbara and Monterey Bay, a distance of over 200 mi (320 km), the only certain breeding locations are the mouths of the Santa Ynez and Santa Maria rivers in Santa Barbara County.

Migration routes and winter distribution are largely unknown. No records confirm least terns on the Pacific Coast of South America, and only a few reports suggest the bird's presence along the Pacific Coast in Honduras, Guatemala, and Panama. Several subspecies of least terns gather seasonally in western Mexico, but since their winter plumage is almost identical, they cannot be distinguished visually. No reliable estimates are available of original numbers of California least terns, but they once were considered abundant along the southern California coast. An observer in 1909 describes counting about 600 pairs along a single beach near San Diego.

The California least tern breeding population averaged about 600 pairs between 1973 and 1975. In 1984, a state survey estimated the breeding population at 931 to 1,001 pairs. The size of the Baja California population is unknown.

Threats

Human encroachment is largely responsible for the least tern's historic decline. The Pacific Coast Highway was constructed early this century along previously undisturbed beach, and summer cottages and beach homes were built in many areas. Increasing human use of the beaches disrupted nesting sites at the same time that feeding areas were being diminished by development or pollution.

Nesting and feeding habitat in the vicinity of most existing colonies can potentially be restored or expanded. Wildlife managers are focusing on recovering degraded coastal wetlands, creating nesting islands, and protecting nesting colonies from excessive human disturbance and predation. In addition, nesting sites are often protected by warning signs or fences.

Alternate nesting sites can be constructed in areas where currently used sites are highly vulnerable to disturbance or are jeopardized by habitat loss. In some wetland areas, improving tidal circulation is essential to restoring fish populations.

The California least tern is also threatened by predators, especially the red fox (Vulpes vulpes ). During the 1988 breeding season, 75% of the terns nesting in three Orange County colonies were killed by foxes. Trapping efforts at the Seal Beach National Wildlife Refuge were overwhelmed by a large influx of foxes.

Conservation and Recovery

The conservation of the California least tern requires the strict protection of its coastal breeding habitats from disturbances associated with humans, such as development along shorelines and predation by domestic and wild animals. It is also essential that the shallow-water feeding habitats are protected from dredging, filling, and water pollution. In many cases, the critical nesting habitats are publicly owned. All such habitats should be strictly protected from development and other threatening activities. Privately owned habitats should be protected by acquiring them and establishing ecological reserves, or by negotiating conservation easements with the owners. In many cases, the nesting habitat will have to be actively managed to reduce the impacts of human disturbances and mammalian predators. The populations of the California least tern should be monitored, including in the Mexican range, and research undertaken into its biology and habitat needs.

Contact

Regional Office of Endangered Species
U. S. Fish and Wildlife Service
Eastside Federal Complex
911 N. E. 11th Ave.
Portland, Oregon 97232-4181
Telephone: (503) 231-6121
http://pacific.fws.gov/

References

Anderson, W. 1970. "The California Least Tern Breeding in Alameda and San Mateo Counties." California Fish & Game 56 (2): 136-137.

Chandik, T., and A. Baldridge. 1967. "Nesting Season, Middle Pacific Coast Region." Audubon Field Notes 21 (5): 600-603.

U. S. Fish and Wildlife Service. 1980. "California Least Tern Recovery Plan." U. S. Fish and Wildlife Service, Portland.