Sterna caspiaCaspian tern

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Geographic Range

Caspian terns have a cosmopolitan distribution, they are found on all continents except Antarctica. They are found along coastlines of oceans, seas, large lakes, and rivers. They migrate between breeding and wintering ranges for the most part, although some populations are resident year-round. (Cuthbert and Wires, 1999)

In the Americas, Caspian terns breed along coastal and inland waterways from the Gulf of Mexico and Baja California northwards through the Great Lakes and Canadian interior and as far north as southern Alaska on the Pacific coast and the Canadian maritime provinces on the Atlantic coast. They winter from southern California to Guatemala along the Pacific coast, including the Gulf of California, and from southern North Carolina on the Atlantic coast to Panama and Venezuela, including the Gulf of Mexico. They also winter in the Antilles. (Cuthbert and Wires, 1999)

Caspian terns breed in coastal areas from Scandinavia to the Baltic and Black Seas, throughout central Asia to Mongolia and the Persian Gulf and Red Sea to southeast Asia. They also breed throughout Australasia and Africa. Winter ranges are in warmer areas of their breeding range, including the Baltic, Black, and Mediterranean Seas, the Persian Gulf, and along African coastlines to South Africa. (Cuthbert and Wires, 1999)

Habitat

Caspian terns are found in coastal habitats, including beaches, marshes, estuaries, or in open habitats on islands in large bodies of water. They forage over water and nest on sandy, muddy, or pebbly shores or areas with little vegetation on islands. Nesting on islands minimizes risks of predation to eggs and nestlings. They migrate along similar habitats as in their wintering and breeding ranges. (Cuthbert and Wires, 1999)

Physical Description

Caspian terns are the largest tern species and are recognized by their large, bright coral red bill and full black cap on the head. Sexes are alike, from 47 to 54 cm long and from 530 to 782 g. Their upperparts are smooth gray and their breast, belly, rump, and tail are white. The primary feathers are dark gray to black on the underside. Their black cap extends to below the eye and onto the back of the head, it can become speckled with white during the non-breeding season and in juveniles. The tail is only slightly notched. They have a robust bill that is deep red to orange, sometimes with dark gray mark at the tip. Caspian terns are distinguished from other terns by their large size, substantially larger than most terns and about the size of gulls. (Cuthbert and Wires, 1999)

  • Sexual Dimorphism
  • sexes alike
  • Range mass
    530 to 782 g
    18.68 to 27.56 oz
  • Range length
    47 to 54 cm
    18.50 to 21.26 in

Reproduction

Caspian terns are monogamous, with pairs staying together at least for the breeding season. Some pairs remain together for many years, but only 25% of pairs mate again the next year in some populations. Pairs are formed soon after birds arrive on the breeding grounds, although some pairs form during migration or on the wintering range. Males attract mates with a "fish flight." Males capture a fish and then fly with it over a group of terns. Females and males join in the display, flying with the male as he repeatedly passes over the group. He then lands near a female and makes bowing movements with his head. Females may ignore the male, try to steal the fish, or beg for the fish. This display is usually repeated several times before the male will feed the fish to the female, at which point the bond is formed and they copulate. Pairs perform a "high flight" display together to cement the bond, ascending and diving together as they vocalize. Symbolic construction of nest scrapes is also part of the courtship ritual. (Cuthbert and Wires, 1999)

Caspian terns arrive on the breeding grounds from late March to late May. Pairs begin to form nest scrapes soon after they arrive on the breeding grounds. They breed in late May and early June, laying from 1 to 3 buffy, splotched eggs. Eggs are laid every 2 to 3 days in a simple scrape nest and incubation begins immediately with the first egg. Caspian terns have 1 brood yearly. Incubation is from 25 to 28 days and fledging occurs 37 days after hatching. Most individuals don't breed until they are 3 years old, although some attempt breeding in their 2nd year.

  • Breeding interval
    Caspian terns breed once yearly.
  • Breeding season
    Caspian terns breed in late May or June.
  • Range eggs per season
    1 to 3
  • Range time to hatching
    25 to 28 days
  • Average fledging age
    37 days
  • Range age at sexual or reproductive maturity (female)
    2 (low) years
  • Average age at sexual or reproductive maturity (female)
    3 years
  • Range age at sexual or reproductive maturity (male)
    2 (low) years
  • Average age at sexual or reproductive maturity (male)
    3 years

Both parents incubate the eggs and alternate frequently throughout the day. A parent will relieve the other by bringing and offering a fish. Then the incubating parent takes the fish and moves off the nest. Females tend to spend more time caring for eggs and young. Parents protect their young from heat by standing above them to provide shade. Young hatch in the order they were laid and are semiprecocial when hatched, with downy feathers and reliant on the parents for feeding. They remain in or near the nest for about a week after hatching. They are fed fish by parents soon after hatching and they begin to accompany parents on foraging trips within a week or so of fledging. Caspian terns have the longest period of dependency of any tern species. (Cuthbert and Wires, 1999)

  • Parental Investment
  • precocial
  • pre-fertilization
    • provisioning
    • protecting
      • female
  • pre-hatching/birth
    • provisioning
      • female
    • protecting
      • male
      • female
  • pre-weaning/fledging
    • provisioning
      • male
      • female
    • protecting
      • male
      • female

Lifespan/Longevity

As in most other animals, most mortality occurs within a few months of hatching (62% in some areas). Adults have high survival rates and can live more than 26 years in the wild.

Behavior

Caspian terns are ungainly on the ground, waddling to walk. They are powerful and graceful in flight and can dive rapidly to capture fish. Caspian terns roost overnight and nest in groups of just a few individuals to many hundreds. Most Caspian terns are found in relatively small groups, though. Nesting is generally concentrated into relatively predator-free nesting habitats, where there may be many terns densely packed into a nesting colony. They may be active at any time of the day, but most foraging seems to be concentrated in the morning hours. Caspian terns are aggressive, but direct physical interactions are usually avoided. Most aggressive interactions occur around defending their small nesting areas. Interactions are usually accompanied by displays or body posturing and calls. (Cuthbert and Wires, 1999)

Some Caspian tern populations migrate and others are resident. Juveniles tend to overwinter in areas closer to their natal grounds than do adults. The timing of migration varies, depending on the region. In general, they begin migrating south from July to September. Most populations arrive on the breeding grounds in March through May. Caspian terns migrate alone or in groups of up to thousands of individuals. They fly at heights of 80 to 100 meters. (Cuthbert and Wires, 1999)

  • Range territory size
    0.5 to 1.5 m^2

Home Range

Caspian terns defend small areas around their nests in the nesting colony. Nest territories are 0.5 to 1.5 meters squared. However, some pairs nest alone and defend larger territories, even entire small islands. Occasionally individuals will defend small foraging areas. (Cuthbert and Wires, 1999)

Communication and Perception

Caspian terns, like most terns, use a variety of vocalizations. Young begin to call from within the egg and use an "i-i-i" call to beg for food. Caspian terns use various calls to maintain contact, express alarm, advertise that they are bringing fish back to the nest, and to beg. Most calls are hoarse and variations on a "ra" or "rau." During courtship, they make steep dives that produce a soft, buzzing sound with their wings. (Cuthbert and Wires, 1999)

Caspian terns communicate through visual displays and body posturing as well. Aggression is displayed with a "head up" posture, with the head held towards the other tern and the feathers ruffled. The head is then bowed forward, showing off the black cap. Appeasement is conveyed with the head held straight up and wings out. (Cuthbert and Wires, 1999)

Food Habits

Caspian terns eat primarily fish, with some crayfish and insects taken occasionally. They forage by flying above shallow water (0.5 to 5 m deep) at heights of 3 to 30 m, usually along a shoreline. As most terns do, they fly with their heads down, peering into the water, when they see prey, they may hover for a moment before making a sharp dive. They may just skim the surface when they dive or they may almost completely submerge themselves for a few seconds. They usually eat their prey as soon as it is captured but may take some fish back to a nest. They may wash fish before offering it to young and often clean their bill in water after feeding young. Dominant fish prey varies regionally, but includes shiner perch (Cymatogaster aggregata), anchovies (Engraulis mordax), alewife (Alosa pseudoharengus), rainbow smelt (Osmerus mordax), yellow perch (Perca flavescens), rock bass (Amblopites rupestris), jacksmelt (Atherinopsis californiensis), topsmelt (Atherinops affinis), staghorn sculpin (Leptocottus armatus), and juvenile salmon (Oncorhynchus). (Cuthbert and Wires, 1999)

  • Animal Foods
  • fish
  • insects
  • aquatic crustaceans

Predation

Most predation is on eggs and hatchlings, which may be taken by a wide variety of avian, terrestrial, and aquatic predators. Reported predators on eggs and hatchlings include gull species (Larus), great horned owls (Bubo virginianus), common ravens (Corvus corvax), domestic cats (Felis catus), dogs (Canis lupus familiaris), coyotes (Canis latrans), red foxes (Vulpes vulpes), striped skunks (Mephitis mephitis), raccoons (Procyon lotor), northern pike (Esox lucius), and western diamondback rattlesnakes (Crotalus atrox). Adult Caspian terns may fall prey to avian predators, such as bald eagles (Haliaeetus leucocephalus) and terrestrial predators when roosting or on a nest (Canis latrans, Vulpes vulpes). (Cuthbert and Wires, 1999)

When a predator approaches a nesting colony, Caspian terns raise an alarm call and will often join together to mob the predator. They are aggressive and will chase any large bird that is close to a colony. Their diving attacks can be very effective, resulting in bloody wounds. However, their habit of taking flight to mob a predator may also leave eggs and nestlings vulnerable. Predators have been observed taking advantage of terns flying to grab eggs and nestlings from exposed nests. Chicks simply crouch in the nest scrape, and are cryptically colored, but will be detected by predators using scent or warmth to find prey.

  • Anti-predator Adaptations
  • cryptic

Ecosystem Roles

Caspian terns are predators of small or young fish in coastal areas, they may be especially important predators in areas near breeding colonies. Caspian terns must compete for limited nesting habitats, including competing with gull species (Larus occidentalis, Larus glaucescens, and Larus argentatus). Parasites of Caspian terns include lice (Actornithophilus funebre, Degeeriella praestans, Menophon, and Philopterus melanocephalus) and cestodes (Dibothriocephalus oblongatum, Schistocephalus solidus, and Paricterotaenia species), trematodes (Diplostomum, Cotylurus, Ornithobilharzia, Clinostomum, and Stephanoprora species), and a nematode (Cosmocephalus species). (Cuthbert and Wires, 1999)

Mutualist Species
Commensal/Parasitic Species
  • nematode (Cosmocephalus species)
  • lice (Actornithophilus funebre)
  • lice (Degeeriella praestans)
  • lice (Menophon)
  • lice (Philopterus melanocephalus)
  • cestodes (Dibothriocephalus oblongatum)
  • cestodes (Schistocephalus solidus)
  • cestodes (Paricterotaenia species)
  • trematodes (Diplostomum species)
  • trematodes (Cotylurus species)
  • trematodes (Ornithobilharzia species)
  • trematodes (Clinostomum species)
  • trematodes (Stephanoprora species)

Economic Importance for Humans: Positive

Caspian tern eggs were once collected for food. They are colorful and fascinating members of native coastal faunas worldwide. (Cuthbert and Wires, 1999)

  • Positive Impacts
  • food

Economic Importance for Humans: Negative

There are no known adverse effects of Caspian terns on humans.

Conservation Status

Caspian tern populations have declined in some parts of their range, especially in Europe and Africa where some populations have been extirpated. Populations in North America have largely increased because of measures taken to protect breeding areas and habitat. However, they are considered threatened in some states, including Michigan. They are considered "least concern" by the IUCN Red List because of their large geographic range and population sizes. (Cuthbert and Wires, 1999)

Other Comments

Caspian terns are sometimes recognized as Hydroprogne caspa. They are sometimes also recognized by the synonym Sterna tschegrava. Molecular data suggests that Caspian terns should be placed in the genus Sterna. (Cuthbert and Wires, 1999)

Contributors

Tanya Dewey (author), Animal Diversity Web.

Glossary

Australian

Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.

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Ethiopian

living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.

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Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

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Neotropical

living in the southern part of the New World. In other words, Central and South America.

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Palearctic

living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.

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acoustic

uses sound to communicate

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

carnivore

an animal that mainly eats meat

chemical

uses smells or other chemicals to communicate

coastal

the nearshore aquatic habitats near a coast, or shoreline.

colonial

used loosely to describe any group of organisms living together or in close proximity to each other - for example nesting shorebirds that live in large colonies. More specifically refers to a group of organisms in which members act as specialized subunits (a continuous, modular society) - as in clonal organisms.

cosmopolitan

having a worldwide distribution. Found on all continents (except maybe Antarctica) and in all biogeographic provinces; or in all the major oceans (Atlantic, Indian, and Pacific.

cryptic

having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.

desert or dunes

in deserts low (less than 30 cm per year) and unpredictable rainfall results in landscapes dominated by plants and animals adapted to aridity. Vegetation is typically sparse, though spectacular blooms may occur following rain. Deserts can be cold or warm and daily temperates typically fluctuate. In dune areas vegetation is also sparse and conditions are dry. This is because sand does not hold water well so little is available to plants. In dunes near seas and oceans this is compounded by the influence of salt in the air and soil. Salt limits the ability of plants to take up water through their roots.

diurnal
  1. active during the day, 2. lasting for one day.
endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

estuarine

an area where a freshwater river meets the ocean and tidal influences result in fluctuations in salinity.

food

A substance that provides both nutrients and energy to a living thing.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

migratory

makes seasonal movements between breeding and wintering grounds

monogamous

Having one mate at a time.

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

oceanic islands

islands that are not part of continental shelf areas, they are not, and have never been, connected to a continental land mass, most typically these are volcanic islands.

oriental

found in the oriental region of the world. In other words, India and southeast Asia.

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oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

piscivore

an animal that mainly eats fish

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

social

associates with others of its species; forms social groups.

tactile

uses touch to communicate

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

terrestrial

Living on the ground.

territorial

defends an area within the home range, occupied by a single animals or group of animals of the same species and held through overt defense, display, or advertisement

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

visual

uses sight to communicate

young precocial

young are relatively well-developed when born

References

Cuthbert, F., L. Wires. 1999. Caspian Tern (Sterna caspia). The Birds of North America Online, 403: 1-20. Accessed April 22, 2009 at http://bna.birds.cornell.edu/bna/species/403.