Sterna elegans

Geographic Range

During the winter months, elegant terns ( Thalasseus elegans ) range along the Pacific coast from Nayarit, Mexico to as far south as Puerto Montt, Chile. The breeding range of elegant terns extends throughout southern California, Baja California, and the Gulf of Mexico, with five known breeding colonies. Isla Rasa in the Gulf of California, Mexico is the largest colony, containing approximately 90 to 97% of breeding individuals. The smaller colony is found in the southern San Diego Bay, California at the Western Salt Company. Other small colonies breed in Bolsa Chica Ecological Reserve, California; almost three hundred pairs were found breeding at Isla Montague in the Gulf of California; as early as 1998 a colony of 3,000 pairs were established in the Los Angeles Harbor, California. In summer, after breeding season ends, elegant terns migrate north from the breeding colony at Isla Rasa, Mexico to central and northern California. Migrations to Oregon and south Washington occur, but are less frequent. Non-breeding ranges of elegant terns range from central California south to Costa Rica all along the Pacific coast.

• Biogeographic Regions
• nearctic nearctic
  • native native
• neotropical neotropical
  • native native

Habitat

Elegant terns are only found off coasts on isolated beaches and on the islands of Isla Montague and Isla Rasa in the Gulf of California, where over 90% of individuals breed. Elegant terns also occupy estuaries when hunting or foraging for food. The beaches they occupy are flat and open, with mudflats available for them to use for roosting. There is little to no vegetation around their habitats. Elegant terns in San Diego Bay breed in ditches above the water created from muddy, salty sediment left behind from evaporated ponds. Non-breeding and migratory habitat of elegant terns consists of bays, harbors, lagoons and estuaries.

• Habitat Regions
• terrestrial terrestrial

• Aquatic Biomes
• brackish water brackish water

• Wetlands
• marsh marsh

• Other Habitat Features
• riparian riparian
• estuarine estuarine

Physical Description

Elegant terns are medium-sized sea birds in the family Laridae . Both males and females can grow anywhere from 390 to 420 mm in length and weigh approximately 260 g. They have long thin bills that are yellow to orangish-red in color. Their beaks are sometimes longer in length than their heads. During breeding season, their upper wings and backs are bluish-gray in color, which contrasts with their darker gray outer primaries. The rest of the plumage on their bodies is white with bits of pink on their undersides. Their tails are forked and their legs are mostly black in color with an occasional spotting of yellow and orange. During the breeding season elegant terns have a black cap on top of their heads and a shaggy crest that covers their foreheads and around their eyes. During non-breeding season the remaining black portion of their caps form a mask, beginning around their eyes and running back on top of their heads to nearly the base of their necks. However, their foreheads turn white in the non-breeding season. Juvenile elegant terns have gray crests instead of the shaggy black ones seen in adults.

• Other Physical Features
• endothermic endothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• sexes alike

Reproduction

Not much is known about the nesting and reproduction of elegant terns, but research speculates that their nesting and breeding patterns might be similar to that of Caspian terns, since they are usually found in similar nesting areas and habitats. Elegant terns court and form pairs either during migration or in their nesting colonies. A small group of terns that have found mates are called “clubs”. Clubs form adjacent to nesting colonies. They form anywhere from two to three weeks before eggs are laid. Nesting begins in April when mated pairs dig nests by scraping indentations in the ground. Elegant terns lay eggs within 24 hours of building their nests. Elegant terns require small, isolated nesting areas with other breeding terns or gull species in the vicinity.

• Mating System
• cooperative breeder cooperative breeder

Elegant terns usually lay only one egg per clutch, with 95% of the nests in San Diego Bay containing only one egg per mated pair. The level of parental care is moderate and equal to that that of many other tern and gull species. Parents take turns incubating their eggs. The incubating parent does not move or leave its nest unless switching nesting duties with its mate. Eggs hatch from early April to mid-June. When chicks are born they are covered in downy feathers, which camouflage chicks with the ground of the nesting sites. Chicks are able to walk around 4 days after being hatched and are able to fly at approximately 34 days after hatching, after which they are able to join mobs of adult terns.

• Key Reproductive Features
• iteroparous iteroparous
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• oviparous oviparous

Both male and female elegant terns in a mated pair incubate, but there is currently no data on relative amounts of time each spends on the nest. Parents relieve each other of incubation duties frequently during day. While incubating and brooding, elegant terns rarely leave their nests except to exchange nest duties with their mates. While one parent is nesting the other parent is out foraging. Parents never leave their nests unattended.

• Parental Investment
• male parental care male parental care
• female parental care female parental care

Lifespan/Longevity

The current recorded lifespan of wild elegant terns is approximately 20.9 years, based on a small sample size of the birds and banding studies. No other information is currently known about their lifespan or longevity and more research is required.

Behavior

Elegant terns have stronger flying skills than other species of terns and beat their wings rapidly while foraging. Despite living primarily near coasts, they only briefly swim, bathe, or preen in the water. They are mostly seen floating on the surface or on buoys. Elegant terns only preen after copulation. They sleep with their eyes closed, on their belly with their heads stretched forward and their bills resting on the ground. When they are resting they stay clustered in large groups, sometimes mixed with other flocks of species resting in the same area. On breeding grounds the adults roost at night in a colony, resting shoulder to shoulder. They do not roost in the water but rest on buoys during both the day and night. Elegant terns exhibit intraspecific competition mostly when establishing nesting grounds or when defending their chicks. They attack by locking their bills together or making repeated, rapid strikes with their bills in what is considered a “fencing” motion. They are territorial only with the area immediately surrounding their nests.

• Key Behaviors
• terricolous
• motile motile
• territorial territorial
• social social
• colonial colonial

Home Range

Elegant terns choose nesting areas with the best visibility, such as crests of steep slopes or flat areas.

Communication and Perception

Elegant terns are considered a highly vocal and social species when interacting with others of their kind or going about their daily activities. When looking for nesting sites they are called “screaming masses” and a group of elegant terns that gather en masse and make noise is called a “spirited cacophony". The calls of adult elegant terns are low-pitched and more closely resemble calls of least terns or sandwich terns. Juveniles emit high-pitched squeaks when begging for food. Elegant terns give calls but none are considered songs. Not much else is currently known about calls made outside breeding season or of any call patterns.

• Communication Channels
• acoustic acoustic

• Other Communication Modes
• choruses choruses

• Perception Channels
• visual visual
• tactile tactile
• acoustic acoustic
• chemical chemical

Food Habits

The primary food source for elegant terns are schooling fish species comprised mostly of anchovies (family Engraulidae ), sardines (family Clupeidae ), silversides (family Atherinopsidae ), gobies (family Gobiidae ), and mackerels (family Scombridae ). Northern anchovies ( Engraulis mordax ) are the main species of fish in their diet. When searching for food, elegant terns hover over the water and then dive into the surface when they spot prey. Elegant terns stop at locations called “clubs”, which are gatherings of terns so that they are able to communicate with each other and discover possible foraging sites.

• Primary Diet
• carnivore carnivore
  • piscivore piscivore

• Animal Foods
• fish

Predation

The dense nesting and colony formation of elegant terns reduces the possibility of predation. Elegant terns also minimize predation by breeding in areas with more aggressive tern species. Other bird species are the only predators that elegant terns have, with their eggs and chicks suffering from predation from Heermann’s gulls ( Larus heermanni ) at Isla Rasa. Gull predation causes colonies to compress and causes excessive crowding, which then results in eggs being broken or lost from their nests. Caspian terns at San Diego Bay play a crucial role in keeping away gulls and other birds in the family Laridae at bay. Caspian terns and their chicks tend to be found somewhat nearby the elegant tern nesting sites, resulting in additional protection. In some cases, peregrine falcons ( Falco peregrinus ) have also been observed taking the eggs of elegant terns.

Ecosystem Roles

There is competition for space between the elegant terns and endangered California least terns ( Sterna antillarum browni ) at Bolsa Chica, California. This competition for breeding space is perhaps due to a broad overlap in diet and foraging habits at locations that both species use for breeding grounds. At Isla Rasa, they often settle en masse in colonies with other species such as gull species. They establish territory and dominance by displacing the gulls and destroying their nests. The presence of elegant terns is tied to their predation on different subpopulations of anchovy in California. Northern anchovies ( Engraulis mordax ) occur in 3 subpopulations. Between the years of 1979 to 1983 there was a negative correlation in the abundance of elegant terns in San Diego Bay with the abundance of anchovy. El Niño weather phenomena impact both anchovy and elegant tern populations, causing birds to disperse farther north than usual in search of food. Ecosystem roles such as mutualistic relationships or commensalism have not been studied or observed in elegant tern populations.

Economic Importance for Humans: Positive

Elegant terns are a valuable indicator of size and composition of fish stocks within their foraging ranges during nesting season. The economic importance of elegant terns has currently not been further studied or observed.

Economic Importance for Humans: Negative

There are no known negative economic impacts of elegant terns on humans.

Conservation Status

As of the 1990s, elegant terns have been reduced to 1 major colony at Isla Rasa, Gulf of California which represents 90-97% of the global breeding population. They are currently designated "Lower Risk, Near Threatened" on the IUCN Red List. They are not yet facing a high risk of extinction. In 1995 elegant terns were designated as a "migratory nongame bird of management concern" in the U.S. This list is intended to: "stimulate a coordinated effort by Federal, State and private agencies to develop and implement comprehensive and integrated approaches for the management of selected species of nongame birds deemed to be in the most need of additional conservation actions.”

A sanctuary system was proposed to protect island wildlife in Gulf of California and the Mexican government declared their islands as wildlife refuges in 1978. The threats to coastal habitats near elegant tern breeding colonies has California considering plans to restore the wetlands of Bolsa Chica Ecological Reserve and establish a wildlife refuge in San Diego Bay.

Additional Links

Encyclopedia of Life

Contributors

Arianne Leon (author), California State University, San Marcos, Tracey Brown (editor), California State University, San Marcos.

References

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