This is a female (top two). Adult males (bottom) have a solid black back. This is a bird of particular interest to me, having written a research paper many years ago 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 polyphyletic 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. Canon PowerShot SX50 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.