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Conservation Status:  Endangered - Protected

Land & AquaticCalifornia Least Tern

Sterna antillarum browni Birds

Aquarium of the Pacific - Online Learning Center - Species Print Sheet

California Least Tern
Least Tern chick with a parent | Courtesy of USFWS
California Least Tern
Least Tern chick with a parent | Courtesy of USFWS

Species Overview

California Least Terns are in the gull family, Laridae. The smallest of the North American terns, they are migratory seabirds that breed primarily along the California coast. Their swallow-like flight gave them their earlier common name, Sea Sparrows. Their flight is very light, graceful, and buoyant. When it increases in speed, their wing beats are so rapid they cannot be counted, in contrast to the slower ones of larger terns that can be.

Species In-Depth | Print full entry

At the Aquarium

The California Least Tern is illustrated on The Wave’s mosaic tile mural, Rios de la Vida (Rivers of Life). The fountain, mural, and accompanying graphics illustrate the story of our Los Angeles and San Gabriel Rivers.

Since 2001 the Aquarium of the Pacific has partnered with the US Navy, US Fish & Wildlife Service, and the Friends of the Seal Beach National Wildlife Refuge to annually restore a 13 acre electrified fence enclosed nesting habitat for the migratory California Least Terns, an endangered species. The refuge is located eight miles from the Aquarium on the US Naval Weapons Station Seal Beach.

Geographic Distribution

Breeding: San Francisco Bay and a few areas along the California coast from San Luis Obispo to San Diego Counties, including southern California’s Seal Beach, San Pedro Bay, Camp Pendleton, and Ballona Creek. Also extreme northern Baja California, Mexico Winter: locations unknown but may be in western Mexico or Central America.

Habitat

California Least Terns nest on beaches, mudflats, and sand dunes, usually near shallow estuaries and lagoons with access to the near open ocean. They roost on the ground in unprotected areas of the coastal environment.

Physical Characteristics

Adults have short, deeply forked tails and short yellowish legs. They have a distinctive black cap with a black blaze or triangle formed by stripes running from the cap across the eyes to the beak, and a white forehead and underparts. Their backs and tops of their wings are pale gray all the way to the tail. The outer edges of their wings are black. The bill is yellow to golden with a black tip. Fledglings are brownish-gray in color with dark bills. Their head caps are not well defined, blending dark gray into the white frontal area.

Size

This species is the smallest North American tern. California Least Terns are 21-23 cm (8.25-9.0 in) from beak to end of tail, with a wingspan of 48-53 cm (19-21 in).

Diet and Feeding

Primary foraging sites for these opportunistic feeders are shallow estuaries and lagoons where they look for small fishes such as anchovy, smelt, silversides, shiner surfperch, and small crustaceans. They also feed nearshore in the open ocean usually near the mouths of lagoons and bays and in freshwater ponds. They hover until they spot prey and then plunge into the water to grab a fish without fully submerging.

Reproduction

California Least Terns start breeding when about three years old. Most begin arriving at their breeding sites in late April. Courtship is an elaborate ritual that usually takes place near an exposed tidal flat or beach. In a ritual called the fish-flight display, a male flies around with a small fish in his beak, often pursued by a female looking for a fishing mate. The chases are spirited and vocal as the birds weave high in the sky and then make paired aerial glides, descending swiftly in close unison. There is also courtship on the ground. The male carries a fish in his beak as he struts around a female in a dance. If she accepts him as a mate, she joins the dance. Pairs are seasonally monogamous.

Colonies consist of 25-50 pairs. Nests, simple scrapes in sand or small gravel substrates, are located in barren to sparsely vegetated places. Usually two eggs are laid in May but the number varies from one to four. The eggs are incubated for about three weeks by both the male and female and both care for the chicks. The chicks are semi-precocial, (hatch with eyes open, down covered, capable of walking soon after hatching). If predation is a problem, adults will try to lead their flightless chicks to a safer location. Chicks fledge at four weeks of age.

A second wave of nesting occurs in June. It is believed that this wave of mating is of younger adults that leave the south later than the older birds or because of a failure of the first nesting. By August, California Least Terns are all headed to Central America.

Behavior

These birds fly over water in a hunchbacked position with the bill pointing slightly downward. They are gregarious and very vocal, especially in nesting colonies. They migrate, roost, and nest in colonies.

Juveniles accompany their adults to post-breeding foraging areas where the water is calm and the food supply is good. There they learn fishing skills and increase their fat storages for the long migration south.

Adaptation

The color of the sand and fine gravel on which these terns lay their eggs varies from nesting site to nesting site and the color of the eggs, and the down color of chicks varies to match the sand. They can be very pale blond, speckled brown or gray, and even reddish where the sand is reddish to provide camouflage from predators. This can also be a disadvantage in that the eggs and chicks are so difficult to see that human beach-goers often step on them when the nesting area is not fenced.

Longevity

Based on observations of banded birds, these terns seem to live 6-10 years. However, some researchers think that many older birds may not make the long migration north from their Central America wintering grounds, since banded birds 17-20 years old have been observed in nesting colonies.

Conservation

California Least Terns are protected under the multi-nation Migratory Bird Treaty. They were also listed as federally endangered in 1970 and as State of California endangered in 1971. The construction of the Pacific Coast Highway in the early 20th century resulted in the direct destruction of shorebird nesting habitat in addition to making these areas more accessible to human encroachment and disturbance of nests and chicks. Species such as the California Least Tern and Western Snowy Plover that nest openly on the sand were especially impacted. Reduction of suitable breeding grounds as a result of housing developments has driven terns to nesting on mudflats away from the ocean and in man-made landfills where they are more vulnerable to predation by domestic and feral dogs and cats and wild land mammals such as red foxes, raccoons, and coyotes. Their fishing grounds have been reduced as a result of dredging, development, and pollution.

Habitat management today focuses on protection of nesting sites from predators and human disturbances. Various nesting sites along the coast are monitored primarily by volunteers.

Amazing Facts

The US Navy, Marine Corps, and US Fish and Wildlife Service have teamed up on California’s coast to manage a large number of breeding sites on military lands. Because these places are usually off-limits to the public, human impact is minimized. Today because of this protection, over one-third of California Least Tern populations breed on US military bases with the largest at Camp Pendleton near Oceanside, California.