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 wing feathers are dark gray to black on the underside. Their black cap might be speckled with white during the non-breeding season and in juveniles. The tail is only slightly notched. They have a large bill that is deep red to orange, sometimes with dark gray mark at the tip.
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. 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.
Caspian terns are found in coastal areas, 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.
Caspian terns form mated pairs that stay together 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. 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.
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 depression scraped in the ground and incubation begins immediately with the first egg. Caspian terns have 1 brood yearly. Incubation is from 25 to 28 days and young begin to fly at 37 days after hatching. Most individuals don't breed until they are 3 years old.
Both parents incubate the eggs. A parent will bring a fish and offer it to their mate on the nest, then take over incubating. 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 are semiprecocial when hatched, with downy feathers but relying on their 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 learning to fly.
As in most other animals, most deaths occur within a few months of hatching. Adults have high survival rates and can live more than 26 years in the wild.
Caspian terns are awkward 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. They prefer to nest in places where there are fewer predators, especially islands. Nesting colonies can be very dense with terns in areas that are safest from predators. They may be active at any time of the day, but most foraging seems to be concentrated in the morning hours. Caspian terns aggressively defend the small area around their nest. Some Caspian tern populations migrate and others are resident. 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.
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)
Caspian terns, like most terns, use a variety of calls. 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 sound like "ra" or "rau." During courtship, they make steep dives that produce a soft, buzzing sound with their wings. Caspian terns communicate through visual displays and body posturing as well.
Caspian terns eat mainly fish, with some crayfish and insects occasionally. They forage by flying above shallow water, 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. Fish prey includes shiner perch, anchovies, alewives, rainbow smelt, yellow perch, rock bass, jacksmelt, topsmelt, staghorn sculpin, and juvenile salmon.
Most predation is on eggs and hatchlings, which may be taken by a wide variety of predators. Reported predators on eggs and hatchlings include gulls, great horned owls, common ravens, domestic cats, dogs, coyotes, red foxes, striped skunks, raccoons, northern pike, and western diamondback rattlesnakes. Adult Caspian terns may fall prey to birds of prey, such as bald eagles, and terrestrial predators when roosting or on a nest, such as coyotes and red foxes. 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 sometimes grab eggs and nestlings from exposed nests when the adults have flown away. Chicks crouch in the nest scrape and difficult to see, but will be detected by predators using scent or warmth to find prey.
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. Caspian terns are parasitized by lice and internal worms.
There are no known adverse effects of Caspian terns on humans.
Caspian tern eggs were once collected for food. They are colorful and fascinating members of native coastal faunas worldwide. (Cuthbert and Wires, 1999)
Caspian tern populations have declined in some parts of their range, especially in Europe and Africa where some populations have gone extinct. Populations in North America have increased because measures were taken to protect breeding areas and habitat. 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.
Tanya Dewey (author), Animal Diversity Web.
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.