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Editor’s Note: Study of the mitochondrial DNA of terns, along with their plumage characteristics, have suggested that the heretofore broadly defined genus Sterna is paraphyletic. Reclassification of this genus now places Caspian Tern in the genus Hydroprogne. See the 47th Supplement to the AOU Check-list of North American Birds for details. Future revisions of this account will account for this change.
The largest tern, the Caspian Tern is distinguished by its massive coral red bill, hoarse vocalizations, and similarities to large Larus gulls in flight. Bent (1921: 202) described it as “king of all the terns . . . the dominant ruling spirit in the colonies . . . the largest, strongest and fiercest of the terns.” Its broad, expansive wings allow it to soar and wheel dramatically like a gull, and its slow wing-beats power a graceful flight that is strong and swift. Its loud raucous call is heard frequently in breeding colonies, especially when nesting birds are approached by intruders.
With the exception of Antarctica, the Caspian Tern occurs on all continents, breeding and/or wintering along coastlines and inland along rivers, lakes, and marshes. In North America, it breeds at scattered localities along the Pacific, Atlantic, and Gulf Coasts, and inland in the western interior, Prairie Provinces of Canada, and along the Great Lakes. It generally nests in colonies. Flat rocky islands, beaches, and sandy shores, sparsely vegetated and littered with driftwood, are typical breeding habitat for this species. Its nests range from mere scrapes in sand or gravel to clam shell or vegetation lined depressions with elaborately built-up rims that contain mollusk shells or crayfish appendages. Changing water levels often lower nesting success, as does competition with gulls and harassment by predators and humans. Birds desert colonies readily if disturbed early in the breeding season.
Caspian Terns from North America winter along the southern portions of the Atlantic and Pacific Coasts and the Gulf Coast, as far south as Colombia and Venezuela.
This species lives almost entirely on fish, capturing them in shallow surface waters near breeding and wintering sites. When fishing, flies with bill pointing downward; spotting a fish, it hovers, flexes its wings and plunge-dives, usually submerging completely.
Populations of this tern have increased in most parts of its North American range, owing to protection of the species and its traditional nesting sites, and to human alterations in habitat. In Europe and Africa, however, it has declined and is now extinct or rare in parts of its former range (Cramp 1985).
In North America, the Caspian Tern has been studied mainly during the breeding season in freshwater and marine habitats. Information on phenology, nesting, parental behavior, mate and site fidelity, habitat, and diet is available from studies conducted in the Great Lakes and on the Pacific and Gulf Coasts (Baltz et al. 1979, Quinn 1980, Clapp et al. 1983, Cuthbert 1985a, 1988, Ewings et al. 1994). Band recovery data have illuminated movement patterns and overwintering sites for Great Lakes and Pacific Coast populations (F. E. Ludwig 1942, J. P Ludwig 1965, Gill and Mewaldt 1979, 1983).