Systematics

In 1834, Thomas Nuttall first used the name Sterna forsteri for this species in honor of Johann R. Forster, a German naturalist. Nuttall’s name was a substitute for an earlier description of this species by Sir John Richardson in 1832 as a variant of Sterna hirundo (a name already occupied for Common Tern). The specimen on which Richardson based his description was probably collected by Thomas Drummond in 1827 along the Saskatchewan River, about 16–80 km west of Cumberland House, Saskatchewan. John J. Audubon applied the name Sterna havelli to this species in 1840, but his illustration depicts a winter plumage Forster’s Tern, an error later corrected by Coues (1863).

Individuals breeding in w. and interior North America said to average slightly larger (especially in wing and tail length) and darker on upperparts (especially mantle) than those breeding along Atlantic and Gulf coasts; these 2 breeding groups have been named separate subspecies, nominate S. f. forsteri and S. f. litoricola, respectively (see Oberholser 1938, 1974). No comprehensive study exists addressing this variation, e.g., spectrophotometric analysis of mantle color accounting for wear and age of specimens or broad geographic analysis of size using birds from known breeding localities. While some authors have maintained Oberholser’s distinctions as valid (see Hellmayr and Conover 1948 and K. C. Parkes in Levine 1998), others (e.g., Am. Ornithol. Union 1957, Olsen and Larsson 1995) consider the species monotypic. Genetic heterozygosity about 4.0% based on 25 individuals assayed from sw. Louisiana (Hackett 1989).

No explicit phylogenetic study addressing relationships among terns, subfamily Sterninae. Forster’s Tern grouped with several similar Sterna terns called “typical black-capped terns” by Moynihan (1959). Phenetic analysis clustering morphological characters (external and skeletal) within Laridae found broad similarity among these terns, with “best estimate” clustering Forster’s Tern nearest Common, Antarctic (S. vittata), Arctic, and Roseate terns (Schnell 1970: Fig. 29). Comparison of allozymes examining broader relationships with-in Laridae, but including all tern species listed above except Roseate Tern, found Forster’s Tern genetically most similar to Royal Tern (S. maxima), these 2 considered sister taxa among those sampled, even though quite dissimilar morphologically (Hackett 1989). Snowy-crowned Tern (S. trudeaui) of South America resembles Basic plumage Forster’s Tern, and these 2 species often thought closely related; analysis of skeletal characters by Schnell (1970) often clustered Snowy-crowned and Forster’s terns together.

Hybridization

No definite hybrids known. Apparent hybrid Common × Forster’s Tern breeding with Common Tern and single, mixed pair of Common and Forster’s terns seen in courtship, both in Massachusetts (Berry 2000, I. C. T. Nisbet pers. comm.); purported extralimital Arctic Tern paired with a Forster’s Tern in California and produced chick (banded) which returned following year (Terrill et al. 2000).