Animal Diversity Web

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)

• Biogeographic Regions
• nearctic
  • native
• palearctic
  • native
• oriental
  • native
• ethiopian
  • native
• neotropical
  • native
• australian
  • native
• oceanic islands
  • native

• Other Geographic Terms
• cosmopolitan

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)

• Habitat Regions
• temperate
• tropical
• terrestrial
• saltwater or marine

• Terrestrial Biomes
• desert or dune

• Aquatic Biomes
• coastal

• Other Habitat Features
• estuarine

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)

• Other Physical Features
• endothermic
• homoiothermic
• bilateral symmetry

• 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)

• Mating System
• monogamous

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.

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

• 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)

• Key Behaviors
• flies
• diurnal
• motile
• migratory
• sedentary
• territorial
• social
• colonial

• 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)

• Communication Channels
• visual
• acoustic

• Perception Channels
• visual
• tactile
• acoustic
• chemical

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)

• Primary Diet
• carnivore
  • piscivore

• 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

• Known Predators

  • 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)
  • western diamondback rattlesnakes (Crotalus atrox)
  • bald eagles (Haliaeetus leucocephalus)

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)

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)

• IUCN Red List

  Least Concern
  More information

• IUCN Red List

  Least Concern
  More information

• US Migratory Bird Act

  Protected

• US Federal List

  No special status

• CITES

  No special status

• State of Michigan List

  Threatened

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.

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.