Gray Wagtail is a slender bird of rocky streams and an uncommon visitor to the United States. It constantly bobs its long tail and has a gray back above a bright yellow belly.
Eastern Yellow Wagtail (Motacilla tschutschensis) has a shorter tail and a grayish-olive back and usually stands in open grassy or tundra habitat. Grey Wagtail has a much longer tail, a clean gray back, and is strongly tied to rocky flowing water.
Why they get mixed upBoth can show a gray head, yellow underparts, and a black tail with white outer feathers in Alaska.