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.

The first photo shows the male of the South East Europe subspecies of the Yellow Wagtail named Black-headed Wagtail (Motacilla flava feldegg) for its distinguished black cap.
The 2nd & 3rd photo is very likely a female Motacilla flava flava of western Europe which 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.

Tivat Saline Nature Reserve, Mrcevac, Montenegro, 27 April 2023