As a relative newcomer (or should that be returner) I hope you don't mind me joining this discussion.
Plumpudding - Pied Wagtails don't have any yellow - unless as Johnboy suggests due to staining. Grey wagtails have more yellow as per the rspb link:
http://www.rspb.org.uk/wildlife/birdgui ... /index.aspThe three usual wagtails we get in this country are pied, grey and yellow - the latter only as a summer visitor. I am not sure about white wagtails (Motacilla Alba) having not come across them before. If you put a search into the RSPB website for White Wagtail, they show Pied Wagtail as Motacilla Alba, whereas Wikipedia suggest that Pied is a sub-species of White and show the latin name of pied as M a yarrellii. Birdguides suggest two races of this species (M alba):
"There are two races of this species in Europe, since the wagtails in Britain and Ireland have darker backs and are known as Pied Wagtails. In the White Wagtails of mainland Europe, the back is storm-cloud grey and the black nape and black bib which border the white face don't meet at the side of the neck."
I certainly don't have Johnboy's experience so I hope I have got this right.
Off-subject a bit, I have just seen (about 2pm) about 4 long-tailed tits on my feeders getting very blown about by the Cornish winds this afternoon.
Anne