A wagtail is a type of small bird which moves its tail quickly up and down as it walks. a small European bird that moves its tail quickly up and down when it walks. Any of 7-10 passerine species in the genus Motacilla and the forest wagtail (Dendronanthus indicus) of Asia. Wagtails continually pump their long tail up and down; the forest wagtail wags its entire body from side to side. They inhabit beaches, meadows, and streamsides, nesting on the ground and roosting in trees. Males of the white, or pied, wagtail (M. alba), common across Eurasia, are white and gray or white and black. The only New World species, the yellow wagtail (M. flava), breeds in Alaska and migrates to Asia