The Collaborative International Dictionary of English v.0.48:

Whip-poor-will \Whip"-poor-will`\, n. (Zool.)
   An American bird (Antrostomus vociferus) allied to the
   nighthawk and goatsucker; -- so called in imitation of the
   peculiar notes which it utters in the evening. [Written also
   whippowil.]
   [1913 Webster]