Reproduction and Breeding Success of California Least Terns in Jalisco, Mexico

Abstract

The Least Tern (Sterna antillarum) is broadly distributed in the United States, with three well-defined subspecies: browni is found along the west coast, athalassos in the central U.S., and antillarum along the east coast and the West Indies (Massey 1981). The status of two subspecies described from Mexico, mexicanus from Sonora (Van Rossem and Hachisuka 1937) and staebleri from Chiapas (Brodkorb 1940) is uncertain. Least Terns have disjunct summer and wintering ranges, breeding in northeast, central, and southwest United States south to Baja California and Sonora in Mexico, and on some Caribbean islands. The east coast population (S. a. antillarum) comprises more than 75% of all the individuals (Buckley and Buckley 1984; Clapp and Buckley 1984). The California Least Tern (S. a. browni) is less abundant; the largest concentration recently recorded is in San Diego, California (Massey 1981). It is considered endangered; its populations began to decline early this century, mainly because of habitat loss, disturbance of nesting sites, and predation by domestic and wild mammals (King 1981, Massey 1981, Jehl 1984, Vermeer and Ranking 1984). California Least Terns breed from the San Francisco Bay in California southward to the tip of the Baja California Peninsula and Sonora, Mexico (Russell and Lamm 1978, Massey 1981, Mellink and Palacios 1993); early in the century a breeding colony was recorded in Oaxaca (Binford 1989). It winters along the coasts of western Mexico, south to northern South America (King 1981, AOU 1983). A group of seventy-five wintering Least Terns was found in Colima in 1979; additionally, some banded Least Terns were found in the same area in 1981, and this region is considered the northern limit of its wintering range (Massey 1981). We report here the first recent tropical breeding record of Least Tern in Mexico, from the Cuixmala River in Jalisco, 150 km to the north of Boca de Pascuales, Colima, the northern limit of the wintering range (Massey 1981). Aside from extending the recent breeding records in Mexico more than 1,000 km to the south, we report the only clutch size data for this subspecies from the tropics, and raise questions about its summer and wintering distribution range. STUDY AREA