Systematics

Editor’s Note: Study of the mitochondrial DNA of terns, along with their plumage characteristics, have suggested that the heretofore broadly defined genus Sterna is paraphyletic. Reclassification of this genus now places Caspian Tern in the genus Hydroprogne. See the 47th Supplement to the AOU Check-list of North American Birds for details. Future revisions of this account will account for this change.

Sometimes placed in monotypic genus Hydroprogne (Am. Ornithol. Union 1983). Characters of Hydroprogne described in Ridgway 1919 . More recently merged along with Thalasseus (crested terns) and Gelochelidon (Gull-billed Tern) into genus Sterna on basis of behavioral and morphological evidence (Voous 1960, White 1965, Schnell 1970, Cramp 1985, Britton 1986, Cooper et al. 1992). The name Sterna tschegrava Lepechin is frequently encountered as a synonym for Caspian Tern. This name was suppressed in 1970 by the International Commission on Zoological Nomenclature in favor of Sterna caspia Pallas. Both descriptions were based on birds from the Caspian Sea and were published simultaneously in 1770; only P. S. Pallas officially assigned a binomial, however.

No clear trends in size or plumage. Compared with birds from Europe, the Middle East, Africa, and Australia, North American birds average slightly smaller in most measurements (see Appendix 1; Ridgway 1919, Serventy et al. 1971, Cramp 1985, Olsen and Larsson 1995).

Monotypic. Supposed larger size of American birds distinguished as separate subspecies (S. c. imperator) is not supported by most authorities. Populations along w. and s. Australian coasts (“ yorki ”) and in New Zealand (“ oliveri ”) sometimes recognized as subspecies S. c. strenua on basis of slightly larger bill, though again supposed differences are matched elsewhere in range of species (del Hoyo et al. 1996).

Caspian Tern is allied with crested terns (Thalasseus) on basis of behavior and morphology (Moynihan 1959). Phenetic analysis of morphology suggested that Royal Tern is most similar to Caspian Tern (Schnell 1970). However, phylogenetic analysis based on starch-gel electrophoresis, conducted by Hackett (1989), suggested that the Caspian is genetically most closely related to Common (Sterna hirundo) and Antarctic (Sterna vittata) terns. Placement of Caspian Tern in genus Sterna instead of Hydroprogne is supported by these analyses.