Taxonomy, Distribution, and Evolution of Golden Plovers (Pluvialis dominica and Pluvialis fulva)

--A discriminant function analysis of specimen measurements eparates 96% of two forms of Lesser Golden Plover in breeding plumage, previously described as subspecies. The forms breed sympatrically in northwest Alaska and are not isolated by habitat, but the incidence of specimens phenotypically intermediate between the two forms is no higher there than in areas of allopatry. The absence of a cline of intergrades or of a hybrid zone indicates that the forms are reproductively isolated through assortative mating and are separate species. To explain the process of speciation in these very similar forms, I propose that the evolution of genetically controlled and markedly different migration routes (Pluvialis dominica to South America and Pluvialis fulva to the central and western Pacific and Indian oceans) during Pleistocene glacial isolation may have selected against hybrids during subsequent secondary contact of populations. This would lead to adaptations in behavior and plumage, such as the difference in male undertail coverts reported here, that would produce assortative mating and reproductive isolation in sympatry. Thus, the requirements of migration and winter range drive the process of speciation. Received 2 August 1982, accepted 28 January 1983. Two forms of Lesser (or American) Golden Plover have been treated by almost all authors as subspecies, Pluvialis dominica dominica nd P. d. fulva (for example, Peters 1934, American Ornithologists' Union 1957, Bock 1958, Johansen 1958, Bannerman 1961, Vaurie 1964, Mayr and Short 1970, Johnsgard 1981). The elucidation of their status hinges on their distribution and interactions on breeding grounds in northwest Alaska, which, in spite of several published discussions (Conover 1945, Bailey 1948, Gabrielson and Lincoln 1959), remain obscure. I have applied a multivariate statistical approach to a study of museum specimens, with the surprising result that I find no evidence of intergradation or of interbreeding in regions of sympatry. I therefore suggest that separate species status for the two forms is appropriate. The form dominica breeds on arctic and subarctic tundra from Baffin Island in Canada west to northwest Alaska. Fulva breeds from northwest Alaska west to the Yamal Peninsula in Siberia, where its range overlaps that of the Eurasian (or Greater) Golden Plover (Pluvialis apricaria). Wintering ranges of the two American forms are distinct: dominica winters in South America, and fulva winters in southern Asia and on many Pacific islands south to Australia and New Zealand (American Ornithologists' Union 607 1957). Both forms have been collected at several localities in Alaska from Barrow to the coast of the Bering Sea. Fulva is more common south of Bering Strait and dominica predominates nearer Barrow (Conover 1945). The problem in determining ranges, and the primary reason these forms have been considered subspecies, is their close similarity in appearance, especially in breeding plumage. Published descriptions of this plumage rely on characters such as "brighter gold spotting" on the back of fulva and a tendency for dominica to "show more white, especially on forehead and sides of breast" (Prater et al. 1977). In juvenal and adult winter plumages the forms are more easily separated, primarily on the basis of the yellowbuff (fulva) compared with brownish-grey (dominica) coloration on much of the body feathering, but even these qualities will not separate all individuals. Differences in size have long been noted, however, with wing length serving to separate a large proportion, but not all, of the Alaskan birds (Conover 1945). Bailey (1948) noted that "where the two breeding areas come together in Northern Alaska, there are many specimens which can not be satisfactorily identified subspecifically," and some authors have concluded that the two forms interbreed in this region, The Auk 100: 607-620. July 1983 608 PETER G. CONNORS [Auk, Vol. 100 resulting in intergrades where the ranges overlap (Gabrielson and Lincoln 1959, Vaurie 1964). A more thorough analysis of this problem has been stalled by the lack of clear intermediate characters to indicate hybridization, because the parent forms differ only in degree rather than presence or absence of characters. A few authors have considered the forms as distinct species. Portenko (1939) suggested this, but subsequently (Portenko 1981) listed them as subspecies and even suggested that P. apricaria and P. dominica may not be specifically distinct. [This latter view has no apparent support among taxonomists. Mayr and Short (1970) considered apricaria and dominica to comprise a species group, or, at the closest, a superspecies.] Stresemann and Stresemann (1966) and Kinsky and Yaldwyn (1981) assigned full species status to fulva primarily because of apparent differences in the molt and maturation schedules of Iand 2-yr-old birds of the two forms. Because of these differences and the distinct migrations, the Ornithological Society of New Zealand (1980) now lists Pluvialis fulva instead of P. d. fulva. The central questions remain, however. Do dominica and fulva interbreed freely? Are the parent forms sympatric, or does a zone of intergradation connect their ranges in northwest Alaska? If they do not interbreed, what are the selective forces and isolating mechanisms that have separated these very similar forms?