The taxonomy of this species is very complex.
In addition, some races are known to hybridize, leading to extreme difficulties in identifying the subspecies.

In Romania for example, birds looking like male motacilla flava flava except for blackish ear-coverts (varying from pure black to dark grey) and slightly darker crown and nape are often named 'M. f. dombrowskii', but they are considered to reflect interbreeding (hybridization) between M. f. feldegg and M.f. flava.
Intergrades between M.f. feldegg and M.f. flava are highly variable in the colour of the crown, and many birds are intermediate and difficult to place in either 'M.f. superciliaris' or 'M.f. dombrowskii' (3rd photo).

M. f. melanogrisea (1st photo) is similar to M. f. feldegg (and sometimes considered a "form" of M.f. feldegg), but having a white malar stripe and a more green than brown neck/mantle.

1st photo: This is the subspecies M. f. melanogrisea. Histria area, Constanta County, Romania, 21 May 2018
2nd photo: This is the subspecies M. f. feldegg. Vadu area, Constanta County, Romania, 20 May 2018
3rd photo: This is the subspecies M.f. dombrowski or M.f. superciliaris. Celic Dere Monastery area, Tulcea County, Romania, 25 May 2018
4th & 5th photo: This is the subspecies M. f. flava. Histria area, Constanta County, Romania, 21 May 2018