SUBSPECIFIC STATUS OF LEAST TERN POPULATIONS

ABSTRACr.-Analyses of seven bill, leg, wing, and plumage characters measured from 267 museum specimens of adult Least Terns (Sterna antillarum) revealed significant morphological differences between sexes but not among three subspecies (antillarum, athalassos, and browni) currently recognized in North America. While individual morphometric characters sometimes did not overlap between a pair of subspecies, discriminant analysis employing criteria developed from the complete morphometnrc data set correctly classified >90% of antillarum specimens but misclassified 39% and 5 1% of athalassos and browni specimens, respectively. Cluster analysis did not segregate specimens into groups consistent with current subspecific taxonomy. Electrophoretic variation in proteins encoded by 50 loci revealed no genetic distinctions between S. a. antillarum and S. a. athalassos for 22 specimens from four breeding sites on the Texas coast, Rio Grande, and Texas panhandle rivers. These data illustrated continued difficulty in distinguishing endangered and nonendangered forms of the same species. Distinctions proposed in original descriptions of North American Least Tern subspecies are not sufficiently definitive; we recommend reassessment of subspecies within the entire species taxon. Received 20 May 1991, accepted 22 Oct. 1991.