Systematics

None documented within North America. Based on enzyme polymorphisms, no genetic differentiation found among colonies within w. Great Lakes (Burson 1990), this consistent with movements of banded birds among colonies (Haymes and Blokpoel 1978). Larger-scale gene-flow documented by exchange of breeding individuals between Great Lakes and Atlantic Coast colonies (Haymes and Blokpoel 1978, Blokpoel and Courtney 1982, Cuthbert and Timmerman 2001). No genetic studies of isolated populations on Bermuda, Gulf Coast, or s. Caribbean. Mitochondrial DNA restriction fragment profiles of birds from Kamchatka (S. h. longipennis) and Louisiana (S. h. hirundo) had nucleotide divergence of 0.0075, a level of differentiation usually interpreted as between populations and species (Zink et al. 1995). For separation of regional populations into different wintering areas, see Migration: nature of migration in the species, below.

Wing length of North American populations said to average 6.5 mm shorter than w. European birds (Cramp 1985), but data in Table 2 suggest smaller difference. Clinal variation in measurements and color of plumage and bare parts from west to east across Eurasia (Cramp 1985); no variation reported within North America.

Three subspecies following Cramp (1985), 2 occurring in North America (also see Higgins and Davies 1996). Subspecies distinguished only in Definitive Alternate plumage when differences in color of underparts, tail, bill, and leg apparent; slight differences in size not useful for diagnosis.

S. h. hirundo Linnaeus, 1758: Breeds in North America and Eurasia east to Kazakhstan and w. Siberia; winters in South America, Africa, and n. Indian Ocean. Palest race with shortest wing but longest bill; in breeding season, bill red with black tip, legs and feet red.

S. h. longipennis Nordmann, 1835: Breeds in central Siberia east to Gulf of Anadyr and south to ne. China; winters in w. Pacific south to Australia; straggler to w. Alaska (Gibson and Kessel 1997). Compared to S. h. hirundo, underparts darker gray, central 2 pairs of rectrices with paler gray outer webs (Olsen and Larsson 1995), wing longer, bill shorter, bill wholly black (some with reddish base), legs and feet dark reddish brown to black in breeding adults.

Extralimital: S. h. tibetana Saunders, 1876 (n. India to w. Mongolia and e. China); S. h. minussensis (central Siberia) recognized by Il’icev and Zubakin (1990), but included in S. h. hirundo by Cramp (1985).

No explicit phylogenetic study addressing relationships among subfamily Sterninae. Common Tern grouped with several similar Sterna terns called typical black-capped terns by Moynihan (1959). Phenetic analysis clustering morphological characters (external and skeletal) found broad similarity among these species, with best estimate of overall similarity grouping Common Tern nearest to Antarctic Tern (S. vittata), then Arctic, Roseate, and Forster’s Terns (Schnell 1970, fig. 29). Allozyme study examining broader relationships within Laridae found Common and Antarctic Terns identical at 23 protein loci studied, suggesting closer relationship than those of either with Arctic (Hackett 1989). Based on behavioral repertoires, Common and Arctic Terns appear very similar (Cullen 1956), but these 2 species support different species of mallophagan parasites (Ward 1955, Malcomson 1960).

Hybridization

Occasionally with Roseate Tern where ranges overlap in ne. North America and w. Europe (see references in Olsen and Larsson 1995, Gochfeld et al. 1998). In ne. U.S., frequency of recognizable hybrids is about 1/5,000 Common Terns at large colonies, higher at smaller colonies where Roseate Terns marginal (ICTN). Hybrids mate with either parental species and are fertile, raising backcross young. No definite records of hybridization with Forster’s or Arctic Terns.