Eastern White Wagtail 2

Chinese White Wagtail Motacilla a.leucopsis

Initially considered a Black-backed Wagtail Motacilla alba lugens , when found on 16th April 2020, Rockanje, The Netherlands, when compared to Plate 27 (upper left illustration):

Alstrom, Per, Mild, Krister and Biill Zetterstrom, 2003.PIPITS & WAGTAILS of Europe, Asia and North America. Helm, London

Later on the Chinese White Wagtail M.a.leucopsis seemed more likely albeit the grey flanks were considered unusual for this subspecies.Grey flanks however are normal for Black-backed  (lugens) and Baical Wagtail (baicalensis) and hybrids leucopsis x lugens do occur in Japan and Taiwan.

   http://birdingkyoto.blogspot.com/2014/11/white-wagtail-presumed-lugens-x.html

see grey flanks in M.a.baicalensis https://www.radioactiverobins.com/eastern-wagtail-3/