Hello Michal,
your bird looks like a male imo with this sharply demarcated jet-black cap, sharply bleeding on the upper back (? confirmation or correction please. Thanks!), and if thats right, back is too pale of a Pied Wagtail. Or is it a
yarelli female?
Extent of the white wing-bars is (just?) within variation for a bird from the alba range, but the one on the median coverts seems huge/monster imo.
Did you notice the dark centers to some back-feathers in the field, too?
The extent of the dark flanks seems to be enhanced by position of the wings in your last picture, but I agree with you extent and darkness is notable for a bird from Poland.
White fringes to the tertials seems unusually broad too. I think this is more strange for an alba White Wagtail than the broad white fringes to the greater coverts from my experience.
This came into my mind again: why are these "Pied Wagtail look alikes"= Wagtails showing some features of Pied often seen in March/early April? Can these be migrating birds from the w-european coast where hybrids are said to be regular? March/April sees migration from Pied Wagtail through Europe according to Adrians P, Bosman D & Elst J. 2010. White Wagtail and Pied Wagtail: a new look. Dutch Birding 32: 229-250.
Yes, I agree with you and Richard, its an interesting/notable bird and it might well be a bird with some Pied genes in it.
Yes, this post came out more confident than it is intended to be. I hope for more comments. Thanks!