This species' systematics and genetics are extremely confusing.
Literally dozens of subspecies have been described at one time or another, and some 15-20 are currently considered valid depending on which author reviews them.
These subspecies vary mainly in colour and head pattern of breeding males, whereas females and immatures are difficult to identify.

This is the South East Europe subspecies of the Western Yellow Wagtail and is named Black-headed Wagtail (Motacilla flava feldegg) for its distinguished black cap in males.
Females have a dark grey head and very washed-out dirty yellowish below and a white throat (2nd photo).
The Motacilla flava flava of western Europe has a blue-grey head with a white supercilium (line above each eye).
The British subspecies, Montacilla flava flavissima has a yellow head (the male) with a brighter yellow supercilium.

1st photo: A male Black-headed Wagtail
2nd photo: A female Black-headed Wagtail

Raysut Wast Disposal Site, Dhofar Region, Oman, 5 February 2017