Why are Grey Wagtails

… not Yellow Wagtails? I had to look this up, “ The grey wagtail has a very long, black-and-white tail, a yellow rump and a yellow belly. It is grey above with black wings. Males have a grey face with a black throat bib and a white 'moustache'. The similar yellow wagtail has a shorter tail and an olive-green back, and does not have a black bib.” So this is the relatively rare Grey Wagtail.