Geographic variation and taxonomy, very complex. Nine to eleven subspecies currently recognized, depending on the authority. This is a bird of particular interest to me, having written a research paper many years ago (Continental Birdlife 2:37-50, 1981) that temporarily resulted the AOU elevating this bird into a separate species called Black-backed Wagtail (M. lugens). However, genetic work by Voelker (2002) found that lugens and ocularis were paraphyletic with respect to each other. I thought that would support them being separate species but Voelker said (in litt.) that they needed to be reciprocally monophyletic instead. Apparently Voelker follows the genealogical species concept. One problem with reciprocal monophyly is that it is just one sample away from being reversed. Also it fails to distinguish between genes controlling plumage and behavior or neutral genes which are not biologically significant. Canon PowerShot SX60 HS.
References:
Alström, P. & Mild, K. (2003) Pipits & Wagtails of Europe, Asia and North America. Identification and Systematics. Christopher Helm, London.
Badyaev, Alexander V., Daniel D. Gibson, Brina Kessel, Peter Pyle and Michael A. Patten.(2017). White Wagtail (Motacilla alba), The Birds of North America (P. G. Rodewald, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America: https://birdsna.org/Species-Account/bna/species/whiwag
Morlan, J. (1981) Status and identification of forms of White Wagtail in western North America. Cont. Birdl. 2(2): 37-50.
Tyler, S. (2017). White Wagtail (Motacilla alba). In: del Hoyo, J., Elliott, A., Sargatal, J., Christie, D.A. & de Juana, E. (eds.). Handbook of the Birds of the World Alive. Lynx Edicions, Barcelona. (retrieved from http://www.hbw.com/node/57821 on 19 June 2017).
Voelker, G. (2002) Systematics and historical biogeography of wagtails: dispersal versus vicariance revisited. Condor 104(4): 725-739.