Is there an easy way to tell a White Wagtail (m a alba) from a female pied wagtail (m a yarrellii)
I never find them very straight forward. I'm sure there are other features but White Wagtails have uniformly plain light grey back extending down between the wings to the rump. Also they usually have white flanks and a clearly defined white line around the edges of the black bib.
Pied Wagtail have a darker blotchy grey back turning black between the wings over the rump, the flanks are duskier and the black bib isn't defined by a white margin.
These may not be the normally accepted features, but I find they are usually pretty reliable.