The Collaborative International Dictionary of English v.0.48:

Poor-will \Poor"-will`\, n. [So called in imitation of its
   note.] (Zool.)
   A bird of the Western United States (Phal[ae]noptilus
   Nutalli) allied to the whip-poor-will.
   [1913 Webster]

WordNet (r) 3.0 (2006):

poorwill
    n 1: goatsucker of western North America [syn: poorwill,
         Phalaenoptilus nuttallii]