Falco rusticolus

from WordNet (r) 3.0 (2006)

Falco rusticolus
    n 1: large and rare Arctic falcon having white and dark color
         phases [syn: {gyrfalcon}, {gerfalcon}, {Falco rusticolus}]

from The Collaborative International Dictionary of English v.0.48

Gyrfalcon \Gyr"fal`con\ (j[~e]r"f[add]`k'n), n. [OE. gerfaucon,
   OF. gerfaucon, LL. gyrofalco, perh. fr. L. gyrus circle +
   falco falcon, and named from its circling flight; or cf. E.
   gier-eagle. See {Gyre}, n., {Falcon}.] (Zool.)
   One of several species and varieties of large Arctic falcons,
   esp. {Falco rusticolus} and the white species {Falco
   Islandicus}, both of which are circumpolar. The black and the
   gray are varieties of the former. See Illust. of {Accipiter}.
   [Written also {gerfalcon}, {gierfalcon}, and {jerfalcon}.]
   [1913 Webster]