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Editor's Note: White Wagtail (Motacilla alba) has had a complex taxonomic history. In addition to M. a. ocularis, which breeds in w. Alaska, this article contains life-history information for M. a. lugens, formerly the Black-backed Wagtail (long considered a subspecies of White Wagtail but segregated as a separate species at the time  this account was written). New taxonomic information has re-consolidated the two "species" (see Systematics).
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 (former) Black-backed Wagtail, now M. a. lugens (Gloger 1829), does so in the western Aleutian Islands.
Both these forms of the White Wagtail occur near rivers and along sea coasts. M. a. ocularis also inhabits more inland areas and is associated more with human settlements than is M. a. lugens. The latter occurs mostly within one to two kilometers of sea coastline and on oceanic islands. In addition to breeding habitat, the two differ in phenology of nesting, song, and courtship displays.
Differences in phenology of breeding may be caused by the greater dependency of M. a. lugens 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 forms and most likely will lead to the expansion of this wagtails’ range in North America. Many aspects of the biology of this species, 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 Wagtail provide excellent opportunities for comparative studies of the breeding biology of these species in relation to Asian populations.