The Collaborative International Dictionary of English v.0.48:

Kestrel \Kes"trel\ (k[e^]s"tr[e^]l), n. [See Castrel.] (Zool.)
   A small, slender European hawk (Falco alaudarius), allied
   to the sparrow hawk. Its color is reddish fawn, streaked and
   spotted with white and black. Also called windhover and
   stannel. The name is also applied to other allied species.
   [1913 Webster]

   Note: This word is often used in contempt, as of a mean kind
         of hawk. "Kites and kestrels have a resemblance with
         hawks." --Bacon.
         [1913 Webster]

WordNet (r) 3.0 (2006):

kestrel
    n 1: small North American falcon [syn: sparrow hawk, American
         kestrel, kestrel, Falco sparverius]
    2: small Old World falcon that hovers in the air against a wind
       [syn: kestrel, Falco tinnunculus]