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Sterna elegans - Elegant Tern

Elegant Tern  image

Geographic range:

West coast of North and South America

Key features:

Down-curved, long (55 – 71 mm), slender orange/red bill and long, shaggy, trailing black crest and deeply forked tail

Similar species:

Sterna caspia -- Caspian Tern

Habitat(s):

bay (sandy shore), estuary, exposed sandy beaches, protected sandy beaches
 

Primary common name:

Elegant Tern

General grouping:

Seabirds and shorebirds

ITIS code:

176931
 

Geographic Range

Range Description:

Elegant Terns breed on islands in the Gulf of California (90% of the known population on Isla Rasa), along the west coast of Baja California, and near San Diego, California (since 1959). Postbreeding birds commonly occur north to the central California coast from midsummer through fall. They winter along the coast of western South America, from Guatemala to Chile. In California they are seen only on the coast, frequenting estuaries and beaches along the coast in summer and fall.

Habitats

bay (sandy shore), estuary, exposed sandy beaches, protected sandy beaches

Abundance

Relative abundance:

The Elegant Tern is a common post-breeding summer and fall (June-October) visitor along the coast in MBNMS. The number of Elegant Terns appears to be rising, and major population increases appear to coincide with El Nino years. Until around 1986, Elegant Terns had not nested in Southern California for years. Now they breed on a few protected beaches. They were once more widespread on their breeding islands in the Gulf of California but commercial egg harvesting and introduced predators caused populations to decrease so that a survey of Isla Rasa in 1975 estimated 9,400–15,500 individuals. With protection the colony had increased to an estimated 44,000 birds in the 1990s. The total population was estimated at 60,000 breeding individuals in the early 1990s. Presently the Global Population is thought to be 70,500. This shaggy-crested tern has the most restricted breeding distribution of any tern in North America. Formerly just a late summer visitor to the Pacific Coast of the United States, the Elegant Tern was first found nesting in San Diego, California in 1959 and has since expanded northward, while simultaneously disappearing from former nesting sites in Mexico.

Species Description

General description:

The Elegant Tern is a medium-sized, slim tern. Its wings are narrow and angled, it has a powerful flight. The underwing is pale with dark tips on outer primaries. Its short tail is deeply forked and white, with dark edgings sometimes. The adult in breeding plumage has a gray mantle, white breast, belly, and rump, with a pink cast on the belly feathers. The face is white with a black cap and a long, trailing, shaggy black crest (the longest of all the terns) and a head that is more long than round. The bill is long (55-71 cm) and thin with a distinct downward curve and is red to orange and paler at the tip. The sexes are similar but in breeding plumage the male's bill is longer, more slender, and more red/orange; while the female's is shorter and more stout. The eye is black, and so are the legs and feet generally, although adults' legs are rarely red or orange, while hatchlings have pink to orange to greenish yellow legs and feet. The adult in non-breeding plumage is similar, but with a white forehead that darkens to streaky black, as if the cap has receded. Juveniles appear similar to non-breeding adults. Elegant Terns are very similar in appearance to the larger Royal Terns, but can be differentiated by their longer, thinner, down-curved bills and larger crests. They are strong fliers, more graceful in flight and with more rapid wing beats than Caspian or Royal Terns. While hovering they beat their wings rapidly while foraging. When not feeding wing beats are slower. They have spectacular cycles of arching flight and dives during courtship. They will swim for brief periods of time and are less reluctant than the Common Tern to alight on the surface of the water. Chicks >2 d old will swim toward center of nearest pond when approached by human. Their most common call is a distinctive ke-e-e-r dropping in pitch at the end. They also use gak-gak-gak when being aggressive or kurt-kurt-kurt or kup-kup-kup when protecting young. Young give a high-pitched squeak. Young begging for food peep.

Distinctive features:

Medium sized gray, white and black tern with long, narrow, down-curved, orange/red bill and black cap and shaggy, trailing black crest, slim body, short legs, and deeply forked tail

Size:

Length: 15-16 in (390-420 mm)
Wingspan: 34 in (86 mm)
Weight: 7-11 oz (200-325 g) males are heavier

Natural History

General natural history:

The Elegant Tern has a restricted breeding range, nesting in isolated mainland or insular colonies, often mixed with Heerman's Gulls (Larus heermanni) and Caspian Terns (Sterna caspia). They are a coastal species, found in shallow water, bays, and estuaries, and sometimes far out to sea. It is extremely rare inland. They are a social bird, nesting in tight groups and calling frequently when feeding. They generally raise only one chick that remains in its nest scrape a few days and then joins a crèche, a large group of chicks, where it is continued to be fed by its parents. Because foraging skills are difficult to learn in this species, post-fledging parental care lasts at least 5-6 months. After breeding they move north and south along the west coast.

Predator(s):

Gulls, Ruddy Turnstones and possibly coyotes take eggs chicks. Peregrine Falcons will take an adult.

Prey:

The key prey items of the Elegant Tern is the northern anchovy (Engraulis mordax). Other prey species include sardines (Clupeidae), silversides (Atherinidae), gobies (Gobiidae), mackerels (Scombridae), and rarely, crustaceans.

Feeding behavior

Carnivore

Feeding behavior notes:

Elegant Terns are highly social and very vocal, especially when feeding in flocks. They forage primarily in marine or estuarine areas, rarely freshwater. They may be offshore or nearshore in shallow lagoons, bays, estuaries, and harbors. In southern California they may forage more than 8 km from their colony and are know to travel 25 km to obtain a single fish They forage by hovering over shallow water and then plunging into the water after fish. They can transport one fish at a time, crosswise in their bill. They feed most often at dawn or dusk and are often heard calling as small groups depart the colony. They generally forage alone or in groups of 2 to 3, although occasionally flocks of a hundred or more are seen to hover, wing-tips almost touching, over a school of fish, with no aggression observed.

February - June

Migration:

The Elegant Tern is a medium to long-distance migrant, having a post-breeding dispersal. Northward movement appears to be linked to oceanographic conditions and prey supply. Immatures may oversummer within their winter range from Mexico south. Migrants may be seen along the Pacific coast of Costa Rica from April to early June. Since they are seldom seen along the coast of Panama it is thought they may be moving far offshore. They are commonly seen moving along the Pacific coast of Mexico mid-February to April as they return to their breeding grounds. They generally arrive at California breeding colonies in early to mid- March.

March - July

Reproduction:

The Elegant Tern competes with Heermann's Gulls and Royal Terns on Isla Rasa, Mexico, for nesting space, usually on isolated, flat, sandy or rocky areas without much vegetation. At San Diego Bay, CA they compete with Caspian and Forster's Terns; and at Bolsa Ecological Reserves they compete with Caspian and Forster's Terns and also Black Skimmers. Courtship may begin during migration or occur at the breeding colony. Courtship involves both male and female who run side by side with the forward part of their wings extended down and away from their body, with necks stretched upward and bills alternately raised and lowered in unison. The male will also call when presenting fish during courtship as they run in parallel. Spectacular cycles of arching flights and dives are also part of courtship behavior. They nest in tight groups, scraping a shallow recess in the sand and rimming it with debris, often within 20 feet of water, choosing areas with good visibility, such as crests of steep slopes or flatter areas. All colonies are somewhat isolated, semitropical, and generally low, flat, and sandy, with little vegetation. In San Diego Bay they breed on dikes of dredged muddy sediment from salt evaporating ponds. At Isla Rasa, Mexico, they nest on pebbled substrate and among volcanic rocks. The female usually lays one 29 mm egg that is olive buff/pinkish and marked and wreathed with dark brown. The egg is laid within 24 hours after completion of the nest/scrape. It is believed that both parents incubate, as they each have two brood patches. The chicks are semiprecocial, but are inactive upon hatching until their downy feathers dry. They spend a few days with parents, being fed fish and becoming ambulatory by days 2 to 4, and when 6 to 14 days old are led by their parents to join a crèche (group of young birds, up to several hundred, each fed by its parents, but guarded by adults who take turns watching over the chicks) for up to 6 months. The chicks grow rapidly and weigh 190-200 g by age of 26 days (adults weigh 200-325 g). As the chicks become able to fly they follow a parent that is foraging and when a fish is caught beg to be fed. They alight on the water momentarily to be fed and then fly again as foraging continues. A second clutch is attempted only if first clutch is destroyed early in season.

May - October

Migration:

After breeding, (primarily at Isla Rasa, Mexico) juveniles and adults migrate northward together and join the terns from California colonies. Chicks and parents remain in large flocks during early phases of migration. Chicks have an extraordinarily long period of dependence on parents lasting more than 6 months after fledging. Migration continues up the Pacific Coast to San Francisco Bay, Point Reyes, and Del Norte County. Small numbers may continue to British Columbia. During all their migrations they may be found on inshore coastal waters, bays, harbors, lagoons, and estuaries. They will roost on coastal mudflats, sandy beaches, islands, and man-made structures. By October or early November they begin their migration south to wintering grounds in Guatemala south to central Chile. Migration is primarily coastal to far off shore, but they have been seen at the Salton Sea and Arizona. Intermittent surges of hundreds or thousands along north California coasts in El Nino years is probably due to increase numbers of anchovy.

October - February

Feeding:

Important wintering areas include Peru's Paracas National Reserve, the Pacific coast from Nayarit, Mexico to Puerto Montt, Chile. They are common in Ecuador and irregular from Panama northward. Vagrants have been recorded from U.S. Gulf and Atlantic coasts and in Europe.

Monterey Bay National Marine Sanctuary:

The Elegant Tern has a remarkably restricted distribution. Only five recently active colonies are known: Isla Rasa in the Gulf of California (with around 22,500 pairs, 90% or more of the known population), Isla Montague in the Colorado delta, San Diego Bay, Bolsa Chica in Orange County, and Los Angeles harbor (founded in 1998). They were once threatened by widespread egg and guano harvesting at nesting colonies. This is now discontinued at Isla Rasa, as the island is a preserve and is protected by wardens and the presence of researchers. Illegal egg harvesting, the introduction of non-native mammalian predators; entanglement in fishing equipment; and other human disturbance are concerns at other potential breeding sites. Stray dogs killed more than 100 Elegant Tern chicks at the San Diego colony in 1982. The greatest conservation concern stems from the fact that virtually the entire population is concentrated on a single island during the breeding season so that any natural or human-induced disaster there could eliminate a large proportion of the global population. Declines in prey abundance as a result of commercial overfishing is a concern on both breeding and wintering grounds. Other threats include the accumulation of pesticides and environmental contaminants and continued degradation of estuarine ecosystems upon which the species is reliant for food. Egg and tissue samples of Elegant Tern from California found various contaminants, including DDE, but no known negative physiological effects. Rising coastal California water temperatures may also shift northern anchovy distributions and abundance, impacting Elegant Terns. The Elegant Tern is currently listed as a species of special concern in California, and has been designated as near threatened by Bird Life International on behalf of the World Conservation Union (IUCN).