The Collaborative International Dictionary of English v.0.48:

Goatsucker \Goat"suck`er\, n. (Zool.)
   One of several species of insectivorous birds, belonging to
   Caprimulgus and allied genera, esp. the European species
   (Caprimulgus Europ[ae]us); -- so called from the mistaken
   notion that it sucks goats. The European species is also
   goat-milker, goat owl, goat chaffer, fern owl, night
   hawk, nightjar, night churr, churr-owl, gnat hawk,
   and dorhawk.
   [1913 Webster]

WordNet (r) 3.0 (2006):

goatsucker
    n 1: mainly crepuscular or nocturnal nonpasserine birds with
         mottled greyish-brown plumage and large eyes; feed on
         insects [syn: goatsucker, nightjar, caprimulgid]