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Caspian Tern Life History

Habitat

Habitat Shorelines

Caspian Terns breed in a wide range of locales including ocean coasts and barrier islands as well as interior lakes and rivers. Like other terns, they set up their nesting colonies on flat, open areas with little vegetation, which allows them to detect predators from a long distance. These areas are often sandy or pebbly, sometimes strewn with shells and other debris or with scattered, short plants. They readily nest on artificial habitats such as dredge-spoil islands. Often, colonies are near other colonial waterbirds’ nests, including shorebirds, gulls, and other tern species. During migration, Caspian Terns frequent just about any large freshwater body or river inland where they can forage and rest. On ocean coastlines they use islands, beaches, impoundments, rivermouths, and mudflats of estuaries. Unlike the smaller Royal Tern, Caspian does not venture far out to sea when foraging, although during migration they probably cross expanses of the Caribbean Sea and other stretches of saltwater. Wintering Caspian Terns inhabit very similar habitats as migrants, from interior rivers and lakes to ocean coastlines.

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Food

Food Fish

Caspian Terns prey mostly on fish, supplementing their diet with crustaceans such as crayfish and occasionally large insects. To locate prey, they fly above water, between 10 and 100 feet high, and scan the water with bill pointed downward. When they spot prey they dive rapidly, usually submerging the body in the process but sometimes snatching the prey from the water without diving in. They usually consume fish in flight quickly after capture, unless the prey item is intended for the mate or chick. Caspian Terns are kleptoparasites (pirates) at times, chasing other terns species and forcing them to give up their catch. On rare occasions, they scavenge invertebrates or dead fish on beaches, and some have eaten small mammals, birds, eggs, salamanders, mussels, snails, crayfish, flies, and beetles. Known fish prey in North America include chum, coho, chinook, and sockeye salmon, steelhead, staghorn sculpin, topsmelt, jacksmelt, rainbow smelt, Pacific sardine, northern anchovy, shiner perch, alewife, yellow perch, and rock bass.

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Nesting

Nest Placement

Nest Ground

Nest scrapes are made in open, sparsely vegetated flat areas with sand, shells, pebbles, gravel, or dirt. Both male and female select sites and make scrapes.

Nest Description

Most pairs line the nest scrape with dried vegetation and ring it with pebbles, shell fragments, sticks, or other objects found near the scrape. Nests average about 7.7 inches across and 1.6 inches tall, with interior 6.3 inches across and 1.8 inches deep.

Nesting Facts

Clutch Size:1-3 eggs
Number of Broods:1 brood
Egg Length:2.3-2.9 in (5.8-7.3 cm)
Egg Width:1.6-1.9 in (4.2-4.8 cm)
Incubation Period:25-28 days
Nestling Period:1-2 days
Egg Description:Buff, sparingly marked with dark spots and sometimes large irregular blotches.
Condition at Hatching:Eyes open. Covered with down and able to leave nest (usually after several days).
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Behavior

Behavior Aerial Dive (water)

Almost immediately on return to nesting areas, Caspian Terns begin courtship displays. Males capture fish and fly back and forth by groups of other terns, showing off their prize. Small groups of both males and females may follow or chase the male in the air. The male eventually lands next to a female and presents the fish, nodding to her as he does. Receptive females usually accept the fish, sometimes giving a juvenile-like feeding display in which they hunch down, jerk the bill upward several times, and call. Males sometimes repeat this process with several females before pairing occurs. In some cases, Caspian Terns arrive on breeding grounds already paired.

Once partnered, Caspian Terns perform courtship flights over the colony area, calling vigorously and flying up together hundreds of feet in the air, their rise broken several times by downward glides. Both male and female participate in selecting and making the nest, and they may make a dozen or more test scrapes before settling on the nest site. This species appears to be monogamous in its mating system, but occasionally both male and female terns mate with individuals other than their partner. Caspian Terns defend their small territories (less than 5 feet across) vigorously, threatening neighbors that come too close by posturing, puffing up the plumage, raising the crest, opening the bill, calling, or attacking. In some parts of the species’ range, Caspians nest solitarily rather than in colonies. Both adults feed and tend the chicks, and one is always present on the territory, even before egg-laying in most cases. Juveniles follow at least one parent southward on migration; at least some populations migrate nocturnally. Adults continue to feed young on the wintering grounds, probably also demonstrating foraging techniques to young birds during this time.

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Conservation

Conservation Low Concern

Caspian Tern populations and their trends are hard to estimate because many colonies are in remote and inaccessible areas. Best estimates from the North American Breeding Bird Survey indicate populations have remained stable overall. Partners in Flight estimates a global breeding population of 530,000 and rates the species a 10 out of 20 on the Continental Concern Score, indicating a species of low conservation concern. In 2002, the North American Waterbird Conservation Plan estimated a North American breeding population of between 66,000 and 70,000; Partners in Flight recently estimated the continental breeding population at 78,000. Regionally and locally, the species is listed as imperiled in many places. The Canadian population is classified as Vulnerable, and the species is listed as endangered, threatened, or special concern in several U.S. states. Threats to the survival of this species throughout its vast range include hunting, pesticides and other pollutants; disturbance of colonies; and loss of breeding areas to sea-level rise.

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Credits

Cuthbert, F. J. and L. R. Wires (2020). Caspian Tern (Hydroprogne caspia), version 1.0. In Birds of the World (S. M. Billerman, editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.caster1.01

Kushlan, J. A., M. J. Steinkamp, K. C. Parsons, J. Capp, M. A. Cruz, M. Coulter, I. Davidson, L. Dickson, N. Edelson, R. Elliott, R. M. Erwin, S. Hatch, S. Kress, R. Milko, S. Miller, K. Mills, R. Paul, R. Phillips, J. E. Saliva, W. Sydeman, J. Trapp, J. Wheeler and K. Wohl (2002). Waterbird conservation for the Americas: The North American waterbird conservation plan, version 1. Washington, DC, USA.

Lutmerding, J. A. and A. S. Love. (2020). Longevity records of North American birds. Version 2020. Patuxent Wildlife Research Center, Bird Banding Laboratory 2020.

Partners in Flight. (2020). Avian Conservation Assessment Database, version 2020.

Sauer, J. R., D. K. Niven, J. E. Hines, D. J. Ziolkowski Jr., K. L. Pardieck, J. E. Fallon, and W. A. Link (2017). The North American Breeding Bird Survey, Results and Analysis 1966–2015. Version 2.07.2017. USGS Patuxent Wildlife Research Center, Laurel, MD, USA.

Sibley, D. A. (2014). The Sibley Guide to Birds, second edition. Alfred A. Knopf, New York, NY, USA.

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