Unusual nesting of the Caspian Tern

TitleUnusual nesting of the Caspian Tern
Publication TypeJournal Article
Year of Publication1958
AuthorsJr. OSewall Pet
JournalJack Pine Warbler

Throughout its North American breeding range, the Caspian Tern (Hydroprogne caspia) ordinarily nests in colonies of several pairs to many hundreds of pairs. Occasionally the colonies are in close proximity to those of gulls and of other water birds, but they are nonetheless separate and rigorously defended against intrusion. In the Great Lakes region, as in most parts of this country where the Caspian Tern nests, the colonies are usually established on islands. To the best of my knowledge all the Michigan colonies are in such situations. In the light of the above statements, the following discovery is of interest. On July 1, 1957, William E. Southern and I investigated the colonies of Herring and Ring-billed Gulls (Larus argentatus and L. delawarensis) on a point of land extending into Lake Huron from Rogers City, Presque Isle County, Michigan. As the birds enjoyed the protection afforded by the Michigan Limestone Company (Calcite Division), which owns the point and excludes the public, the colonies were thriving. There may have been as many as 300 pairs in the Herring Gull colony and 800 pairs in the Ring-billed colony, the former occupying the outer reaches of the point, the latter the middle section. Hundreds of well-develped young were in evidence everywhere. To our astonishment we noted a pair of Caspian Terns joining the gulls overhead in a group protest at our presence. When we withdrew to watch from a distance, the pair soon settled on the ground where the two gull colonies almost adjoined. Beside the pair stood a Caspian Tern chick! Both parents immediately drove off the neighboring gulls and gull chicks by vocal threats and outright attacks, thus clearing a circle for the family. The chick was already well developed, being able to run fast (as we learned when we caught it for banding) and having a wingspread of about two feet. The nesting of this pair of Caspian Terns is unusual in at least three respects. (1) It is probably the first known instance of the species breeding on the mainland of Michigan, and a rare instance of its nesting in a non-insular location anywhere in North America. (2) The pair was not nesting in a colony of its own species. Apparently the urge to nest in a colony was satisfied by nesting in the immediate vicinity of gull colonies. (3) These two birds managed, through their large size and natural belligerency and without the group defense provided by a colony of their own kind, to rear a chick near Herring Gulls which are larger than Caspian Terns and notoriously predaceous on downy young of terns and other young birds. The nesting of a lone pair of Caspian Terns is not particularly unusual in certain parts of the world. For example, in northern Europe the species not only nests in colonies but also regularly in odd pairs or groups of two or three pairs ("The Handbook of British Birds" by H. F. Witherby and others, 1944). Nor is nesting on the mainland unusual. In northern Europe the species may select shores ("The Handbook of British Birds"); in New Zealand, shingles of rivers ("The Birds of New Zealand" by A. M. Bailey, 1955); in Australia, coastal headlands ("A Guide to the Birds of Australia" by N. W. Cayley, 1947). Such differences in habits are to be expected of a cosmopolitan species with widely separated populations. Where in one country the Caspian Tern can breed successfully only in colonies on islands, in another country, with a different set of ecological factors, it can also breed in lone pairs in a mainland environment.