Current Distribution Rangewide

Found along the Pacific Coast of California, from San Francisco southward to Baja California, and in Chiapas, and Jalisco during the breeding season [1;2]. Wintering likely from Baja California to southern Mexico or along the Pacific coast of Central America and north to at least Colima, Mexico [3 cited from 2].

Known Populations in San Diego County

Occurrences found in Camp Pendleton, Borderfield State Park, San Diego Bay National Wildlife Refuge, Fiesta Island, Flood Control Channel Southern Wildlife Preserve, Mariner's Point, Mission Bay Park, Silver Strand State Beach, Agua Hedionda Lagoon Ecological Reserve, San Luis Rey River Park, Tijuana Slough National Wildlife Refuge, Torrey Pines State Natural Reserve, Batiquitos Lagoon Ecological Reserve, Buena Vista Lagoon Ecological Reserve, Agua Hedionda-SDGE, San Elijo Lagoon Ecological Reserve, Cardiff State Beach, James Scripps Bluffs Preserve, and San Dieguito Lagoon.

List Status

FE, CE

Habitat Affinities

Nest in colonies on relatively open, flat beaches kept free of vegetation by natural scouring from tidal action and along lagoon or estuary margins [1;3 cited from 2]. Forages along seacoasts, beaches, bays, estuaries, lagoons, lakes, and rivers [5 cited from 2]. Rests and loafs on sandy beaches, mudflats, and salt-pond dikes [6 cited from 2].

Taxonomy and Genetics

A subspecies of least terns where two other subspecies, Interior (S. a. athalassos) and Eastern (S. a. antillarum), are recognized in the United States [7 cited from 4]; however, there is little genetic variation among the subspecies which questions the validity of this division [8 cited from 4]. A taxonomic change by the American Ornithologists' Union (AOU) [9 cited from 4] resurrected the genus Sternula for the least tern based on genetic studies.

Seasonal Activity

Fall migration commences the last week of July and first week of August. Several weeks before the Fall Migration, adults and young wander along marine coastlines, congregating at prime fishing sites. In the autumn, adults move south along the california coast with their fledglings stopping to rest and feed along the migration route [2;10 cited from 1].

Life History/Reproduction

Arrive at California nesting sites in late April, where courtship and pair formation activities last into early May [10 cited from 11]. Typical colony size is 25 pairs [1]. Egg-laying and incubation generally begins in mid-May, and pairs may renest following loss of their first clutch [12]. Nests are simple scrapes made on non-vegetated substrates, usually consisting of one to three eggs, and are incubated for 19-25 days by both parents [1;10 cited from 11;13 cited from 4]. Chicks are mobile and leave the nest at only two days after hatching, the semi-precocial young are tended by both parents, and first flight happens at approximately 20 days after hatching [10 cited from 11;13 cited from 4]. Usually start breeding in their third year, but it is not uncommon to start in their second year. Can re-nest up to two times if eggs or chicks are lost early in the breeding season [1;12]. Usually nests in same area in successsive years; tends to return to natal site to nest [14]. Very gregarious and forage, roost, nest and migrate in colonies [1]. A long-lived species and banded birds have been recovered after 24 years [4].

Diet and Foraging

Prey species captured and consumed are remarkably similar across a broad geographic range including the San Fransisco Bay, southern California, and the Gulf of California off Baja Mexico [11;15;16]. Most abundant prey species selected in California are northern anchovies (Engraulis mordax), topsmelt (Atherinops affinis), jacksmelt (Atherinopsis californiensis), deepbody anchovies (Anchoa compressa), and slough anchovies (Anchoa delicatissima). For coastal birds, preference for nearshore foraging habitats is apparently related to prey species selection as northern anchovies, topsmelt, and jacksmelt (all marine species) comprise 92% of captured fish [15].

Dispersal

No information.

Threats

Threats to tern colonies include predation, human disturbance, and nonnative plants. Other threats to terns are food shortage and environmental contamination [1].

Literature Sources