Bøger af R Michael Erwin

The Gull-billed Tern (Gelochelidon nilotica) is a medium-sized tern that breeds in small, scattered, often ephemeral colonies, typically in habitat devoid of vegetation near marine waters or saline lakes. In North America, the species breeds along the Atlantic coast south of New York, the Gulf of Mexico, and the Pacific coast of California and Mexico. Its distribution has contracted from known historic range along the Atlantic Coast, but has expanded along the Pacific Coast. Range changes in Mexico are unknown due to fragmentary knowledge of historical colony locations, but some range contraction may have occurred. Two subspecies (G. n. aranea and G. n. vanrossemi) occur in North America. The current population of G. n. aranea in the United States is estimated to be approximately 3610 pairs, over 60% of which occur in Texas. The number of birds in Texas appears stable, but the number of individuals has declined in Maryland, Delaware, Virginia, North Carolina, Florida, and possibly Georgia. G. n. vanrossemi has 737 to 808 pairs breeding in western Mexico and southern California. Gull-billed Terns are designated as a Bird of Conservation Concern by the U.S. Fish and Wildlife Service. G. n. aranea is designated as endangered, threatened or of management concern in nine states and G. n. vanrossemi is designated as a Bird Species of Special Concern in California. The main causes of population declines in North America are disturbance of nesting colonies, loss of natural nesting islands, and development or modification of upland foraging habitats. This species often nests on artificially deposited substrates, suggesting it could respond to management of breeding habitat. Management priorities for Gull-billed Terns are: (1) protection of known nesting colony sites; (2) enhancement and conservation of potential nesting and foraging areas; (3) predator control; (4) development of population viability models; and (5) resolution of conflicts with other species and aquaculture. Research and monitoring needs are: (1) resolution of the subspecific identity of birds breeding in North America; (2) demographic studies addressing population viability; (3) the identification and linkage of breeding and non-breeding ranges; (4) studies of habitat use and ecology during the breeding and non-breeding seasons, especially in Mexico and Central America; (5) continued monitoring of breeding colonies, particularly in the Gulf Coast of Mexico; and (6) the establishment of monitoring efforts in the West Indies.