White wagtails are Pied wagtails that look a little different, come from a different location and so are usually seen on migration. That’s the gist of it, but trying to nail down the differences between white and regular Pied wags can be tricky. There’s a Bird Guides article online that contains all the facts but to summarise: our regular Pied wagtails that mostly stay put are subspecies Motacilla alba yarrellii, whereas white wagtails are subspecies Motacilla alba alba, and most of the birds seen in Britain are moving from southern continental Europe and Africa to Greenland and Iceland (and vice versa in the autumn). White wagtails can be identified by the clean line between the black at the back of the head and the grey back (especially in male birds); they have a pale grey rump, if you can see it; and the belly and flanks are a very clean white (as opposed to the more streaky grey of Pied wags). Here’s one of several I spotted on Cardiff Bay Barrage yesterday.