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Editor's Note: White Wagtail has had a complex taxonomic history. In addition to White Wagtail, this article contains life-history information for the Black-backed Wagtail, long considered a subspecies of White Wagtail but segregated as a separate species at the time (1996) this article was written. New taxonomic information has re-consolidated the two "species." Future revisions of this
article will clarify this history.
Although the well-known White Wagtail complex includes some of the most thoroughly studied birds in the western Palearctic (e.g., Motacilla alba alba and M. a. yarrellii), two eastern palearctic forms that occur annually in North America (Alaska)—and that are the focus of this account—remain poorly known. The easternmost subspecies of the White Wagtail, M. a. ocularis Swinhoe 1860, reaches the western perimeter of Alaska in the Bering Strait region, and the Black-backed Wagtail, Motacilla lugens Gloger 1829, does so in the western Aleutian Islands.
Originally described as a full species but long treated as a subspecies of the White Wagtail, the Black-backed Wagtail has been segregated again as a full species by the American Ornithologists’ Union (1983) based on work in Asia (e.g., Nazarenko 1968). Breeding widely in remote parts of Asia, these two taxa are among the least studied forms of the Motacilla alba complex.
Both White and Black-backed wagtails occur near rivers and along sea coasts. White Wagtail also inhabits more inland areas and is associated more with human settlements than is Black-backed Wagtail. The latter occurs mostly within one to two kilometers of sea coastline and on oceanic islands. In addition to breeding habitat, the two species differ in phenology of nesting, song, and courtship displays.
Differences in phenology of breeding may be caused by greater dependency of Black-backed Wagtail on availability of flying insects than in other wagtails. A growing number of human settlements, roads, and other industrial constructions along coastlines and in northern regions increases suitable breeding and foraging habitats for both species and most likely will lead to the expansion of the wagtails’ range in North America. Many aspects of the wagtails’ biology, especially territoriality, physiology, and breeding behavior, are better studied in eastern palearctic populations, and the Old World literature sheds much light on the biology of these species. Thus, understudied North American populations of White and Black-backed wagtails provide excellent opportunities for comparative studies of breeding biology of these species in relation to Asian populations.