Systematics

Among the most polytypic of all shorebirds, with up to 9 subspecies recognized (others proposed), based on differences in plumage and morphometrics. American Ornithologists’ Union (1957) recognizes a single North American subspecies, but others (Todd 1953, MacLean and Holmes 1971, Browning 1977, 1991, Cramp and Simmons 1983, Greenwood 1986, Tomkovich 1986) consider 2–3 subspecies for North America. Recent genetic studies (Wenink 1994) are more conservative and recognize only 5 subspecies worldwide, 2 for North America (but see below). Plumage variation most pronounced in dorsal color and pattern (degree of brightness), breast, neck, and undertail streaking, and amount of white on outer primaries. Morphometric differentiation of subspecies most obvious in culmen length, wing length, and mass (see Appendix). Clinal differences in size sequential from smallest race in ne. Greenland (C. a. arctica) eastward and circumpolar to largest in Alaska and central Canada (C. a. pacifica and C. a. hudsonia). Culmen and wing length average 30% and 8% greater, respectively, in C. a. pacifica than in C. a. arctica (Greenwood 1986).

Considerable difference in opinion regarding taxonomic status of North American forms. Todd (1953) first to propose subspecies status of Canadian Arctic (C. a. hudsonia) and n. Alaska (C. a. arcticola) populations based on morphometrics, plumage, and breeding and wintering distribution. MacLean and Holmes (1971) concurred that n. Alaska birds were clearly separable (based on culmen length) from those of w. Alaska (C. a. pacifica) and Canada, but that birds from n. Alaska were indistinguishable from, and thus belonged to, the e. Siberian population (C. a. sakhalina). Browning (1977, 1991) found significant differences in culmen length and plumage between n. Alaska and e. Siberian birds (contra Mac-Lean and Holmes 1971) and concluded that Todd’s (1953) C. a. arcticola was valid. Greenwood (1986), using multivariate techniques, found culmen length, among 6 primary variables, to have the greatest discriminating power in geographic separation of worldwide populations. He recognized C. a. arcticola from n. Alaska, C. a. pacifica from s. Alaska, and C. a. hudsonia from Canada. Despite the morphological similarity, Greenwood (1986) assigned subspecies status to the Canada (C. a. hudsonia) and s. Alaska (C. a. pacifica) stocks based on their mutually exclusive geographic ranges on the continent.

Recent analysis of mtDNA of worldwide populations (Wenink et al. 1993, Wenink 1994) identified only 5 major lineages, including a separate lineage for the far e. Siberian group (C. a. sakhalina) and for 2 North American lineages (C. a. pacifica and C. a. hudsonia). No significant phylogenetic dichotomy in mtDNA region sequence found among birds from n. (n = 15) and w. (n = 7) Alaska, resulting in all Alaska birds being assigned to C. a. pacifica race. Further, no C. a. sakhalina haplotypes found in Alaska breeding birds (n = 22), but 2 of 9 birds wintering along the Pacific Coast of North America possessed such haplotypes, suggesting interchange of birds between Asia and North America (see below).

Other evidence indicates that lineages and movements of Beringian populations may be more complicated than presented by Wenink et al. (1993). Marking studies indicate significant spatial segregation during most of winter. Birds marked in n. Alaska have been recovered in Russian Far East and East Asia (Norton 1971). Morphometric data indicate that 2 distinct groups of Dunlin stage in fall on the Yukon-Kuskokwim Delta of w. Alaska (REG). Further, color-marked birds from there have been seen throughout winter in Asia (Japan-Korea, n = 12), and along the Pacific Coast of North America (n = 37). C. a. pacifica breeds and stages on the Yukon-Kuskokwim Delta, but e. Siberian (C. a sakhalina) and/or n. Alaska (C. a. arcticola) populations also occur there before returning to wintering grounds in East Asia. Marking has also shown that C. a. pacifica breeding on the Yukon-Kuskokwim Delta winter primarily in the Pacific Northwest, whereas C. a. pacifica from the Alaska Peninsula winter in California (REG). Further clarification of Beringian populations based on genetic and marking studies is warranted.

The antiquity of North American populations dates to the late Pleistocene (Wenink et al. 1993). C. a. hudsonia is considered ancestral, having split about 223,000 yr ago; C. a. pacifica and other Beringian clades split as recently as 71,000 yr ago (Wenink et al. in press, contra Wenink et al. 1993).