Systematics

Five geographic races described in the Americas, but descriptions based largely on qualitative taxonomic assessment; few specimens and limited statistical analysis used for original descriptions (Thompson et al. 1992, Patten and Erickson 1996). From extensive examination of body measurements, all subspecies are similar to coastal S. a. antillarum (see Appendix; Thompson et al. 1992), but see further discussion of age, plumage wear, and seasonal movements that may influence taxonomic assessments (Cardiff and Remsen 1994). Massey (1976) found no consistent morphological, behavioral, or vocal differences between coastal Least Tern (S. a. antillarum) and California Least Tern (S. a. browni). Electrophoretic analyses of S. a. antillarum and S. a. athalassos from 4 Texas sites revealed no genetic distinction among populations; extensive overlap of physical characteristics exists among 3 recognized subspecies north of Mexico (Thompson et al. 1992). Chick banded on Texas coast later nested in Kansas (Boyd and Thompson 1985), suggesting interrace exchange over large area. Taxonomy of a subspecies in Mexico is questioned (Palacios and Mellink 1996). Lack of clear differences among populations and evidence of movements among populations suggest that distinctions are dubious and variation is clinal (Olsen and Larsson 1995).

S. a. antillarum breeds in coastal e. North America north to coastal Maine, east to Bahamas and south through West Indies and coastal e. Mexico (Tamaulipas and Yucatán Peninsula) to Belize, n. Honduras, and coastal Venezuela. Known to breed in Jamaica only since 1971 (Alleng and Whyte-Alleng 1993). Interior Least Tern (S. a. athalassos) is from inland North America, primarily Mississippi River and its tributaries (also part of Rio Grande drainage). S. a. browni is from Pacific Coast of California (San Francisco southward) and Baja California (Massey 1977, Carvacho et al. 1989, Garcia and Ceballos 1995). Individuals nesting in se. New Mexico since 1950 (Montgomery 1959, Ligon 1961, Hubbard 1978) not identified to subspecies but in range of endangered S. a. athalassos . S. a. mexicana and S. a. staebleri nest in Gulf of California and south along Pacific Coast to Oaxaca and Chiapas, Mexico, although subspecies assignments are uncertain (Patten and Erickson 1996). Within respective ranges, populations localized and increasingly fragmented, especially in California and interior U.S., where federally listed as Endangered.

Least Tern of North America and Little Tern, a sister species in Europe, Asia, Malaysia, and Australia, include at least 12 described subspecies. Least and Little Terns distinguished primarily by vocal differences (Massey 1976). Old World forms of Little Tern summarized by Cramp (1985). Yellow-billed Tern (Sterna superciliaris) and Peruvian Tern (S. lorata) of South America, Saunder’s Tern (S. saundersi) of nw. Indian Ocean, and Fairy Tern (S. nereis) of Australia appear to be a superspecies with Least and Little terns (Am. Ornithol. Union 1983). Breeding ranges of all these species have little or no overlap; physically these species differ only in minor plumage features and color of bill and legs (Cramp 1985).