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The black on the mantle and dusky-grey on the flanks make it a Pied, M. alba ssp yarrellii.
My Flickr photos...
Could you please give me a bit more detail about how you distinguish the "ssp yarrelli" from other Pied Wagtails. Thanks you,
Sara Garcia Hipolito
It can be simple in some adults and quite difficult in many other ages/plumages. Usually, Pied (yarrellii) has dusky grey flanks and (in males) black on the back, while White (alba) has white flanks and is paler grey on the back. Pied have a black rump compared to grey on White, but that can be hard to judge. It is difficult to describe, but easier to see in pictures. If you google them both, you will start to notice the differences.
This ringers' site is quite technical, but very good for the differences...
Thanks for the description and link
Agree with Ophrys comment.
Jamie from Briantspuddle
Lat/Lng: 54.2935, -8.1551
OS grid ref: NU997061