Geographic variation and taxonomy, very complex. Nine to eleven subspecies currently recognized, depending on the authority. This is the nominate race, sometime considered part of the "white-faced" group. It breeds across much of Europe (except most of the British Isles) and is characterized by a gray back strongly contrasting with the black cap in adult males such as this. Also note the extensive gray sides and flanks. Females usually are much whiter. This individual (top and middle) is in the process of molting chin feathers and will have a whitish throat by winter. Canon SX50 HS PowerShot
References:
Adriaens, P., Bosman D. & Elst, J. (2010) White Wagtail and Pied Wagtail: a new look. Dutch Birding 32: 229-250.
Alström, P. and K. Mild. 2003. Pipits and wagtails. Princeton: Princeton University Press.
Badyaev, A. V., D. D. Gibson, B. Kessel, P. Pyle, and M. A. Patten (2017). White Wagtail (Motacilla alba), version 3.0. In The Birds of North America (P. G. Rodewald, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. Retrieved from Birds of North America: https://birdsna.org/Species-Account/bna/species/whiwag
Morlan, J. 1981. Status and identification of forms of White Wagtails in western North America. Continental Birdlife no. 2:37-50.
Tyler, S. (2018). 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 https://www.hbw.com/node/57821 on 9 September 2018).