At this time of year many species of birds are in transiton from winter to breeding plumage - this can be pretty confusing for the observer. The "Yellow" Wagtail in the photos shows features that field guides describe as indicative of Citrine Wagtail i.e. pale-centered ear-coverts and a pale surround to the coverts. However, the deep yellow underparts, the large grey breast-patches and the fact that the supercilia do not join above the bill are, I think, more features of "taivana" Yellow Wagtail. So, I would agree with Mike on the id of this particular bird.
Breeding males are a lot easier - as previous photos show!!
dave
