any of the 7 to 10 species of the bird genus Motacilla, of the family Motacillidae, together with the forest wagtail (Dendronanthus indicus) of Asia. Wagtails are strongly patterned birds of beaches, meadows, and streamsides; they usually nest on the ground but roost in trees. The birds are so named because they incessantly wag their long tails up and down. The forest wagtail wags its entire body from side to side. Males of the white wagtail (Motacilla alba), common across Eurasia, are variably white and gray or white and black. The variety in Britain is called pied wagtail. The only species reaching the New World is the yellow wagtail (M. flava, sometimes Budytes flavus), which breeds in Alaska and migrates to Asia.