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.
   [1913 Webster]

   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
      (Nemoricola Indica).

   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
      fantail.

   Water wagtail.
   (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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

wagtail
    n 1: Old World bird having a very long tail that jerks up and
         down as it walks