The Collaborative International Dictionary of English v.0.48:
Wagtail \Wag"tail`\, n. (Zool.)
Any one of many species of Old World singing birds belonging
to Motacilla and several allied genera of the family
Motacillidae. They have the habit of constantly jerking
their long tails up and down, whence the name.
Field wagtail, any one of several species of wagtails of
the genus Budytes having the tail shorter, the legs
longer, and the hind claw longer and straighter, than do
the water wagtails. Most of the species are yellow
beneath. Called also yellow wagtail.
Garden wagtail, the Indian black-breasted wagtail
Pied wagtail, the common European water wagtail (Motacilla
lugubris). It is variegated with black and white. The
name is applied also to other allied species having
similar colors. Called also pied dishwasher.
Wagtail flycatcher, a true flycatcher (Sauloprocta
motacilloides) common in Southern Australia, where it is
very tame, and frequents stock yards and gardens and often
builds its nest about houses; -- called also black
(a) Any one of several species of wagtails of the restricted
genus Motacilla. They live chiefly on the shores of
ponds and streams.
(b) The American water thrush. See Water thrush.
Wood wagtail, an Asiatic wagtail; (Calobates sulphurea)
having a slender bill and short legs.