The White Wagtail is a ubiquitous species that occupies a wide variety of open (non-forested) habitats across Eurasia. The species is represented by a complex of nine subspecies that exhibit marked variation in plumage pattern and coloration (gray vs. black) across its immense breeding range. 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), the two eastern Palearctic forms that occur regularly in North America (Alaska) remain poorly known. The easternmost subspecies, M. a. ocularis, reaches the western perimeter of Alaska in the Bering Strait region where it breeds annually in small numbers, and M. a. lugens (formerly the "Black-backed" Wagtail) is a rare, occasional breeding species in the western Aleutian Islands. Both ocularis and lugens occur as vagrants elsewhere in North America.
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 closely 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 the availability of flying insects than in other White Wagtail subspecies. 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 could lead to the expansion of the White Wagtail 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 this species. Thus, understudied North American populations of White Wagtail provide excellent opportunities for comparative studies of the breeding biology of these subspecies in relation to Eurasian populations.