Trends in Caspian Tern Numbers and Distribution in North America: A Review

Abstract

-The breeding range of the Caspian Tern (Sterna caspia) covers large parts of six geographic regions in North America. We obtained data from individuals who monitor colonial waterbirds to determine current status and distribution of the Caspian Tern in the U.S., Canada and Mexico and to report regional population trends. We estimate 32,000-34,000 pairs nested in North America from the late 1980s to 1998. The species' breeding range is expanding along the Pacific coast and in central Canada. Over the last three decades numbers of breeding pairs have significantly increased in the Great Lakes, central Canada, and along the Gulf and Pacific coasts; the Atlantic coast is the only region where significant decline has occurred. On the Pacific and Gulf coasts, large numbers of Caspians utilize artificial habitat (e.g., dredge spoil islands, salt dikes and levees, landfills, artificial lakes). Changes in food supply may be at least partly responsible for population increases in the Great Lakes, on the Pacific coast, and in central Canada in Manitoba. Other factors that may contribute to increases include creation of artificial habitat that Caspian Terns find attractive for nesting, and legislation that protects Caspian Terns and their habitat. Received 15 Mar 1999, accepted 19July 1999.