The Collaborative International Dictionary of English v.0.48:
Ptarmigan \Ptar"mi*gan\, n. [Gael. tarmachan; cf. Ir. tarmochan,
tarmonach.] (Zool.)
Any grouse of the genus Lagopus, of which numerous species
are known. The feet are completely feathered. Most of the
species are brown in summer, but turn white, or nearly white,
in winter.
[1913 Webster]
Note: They chiefly inhabit the northern countries and high
mountains of Europe, Asia, and America. The common
European species is Lagopus mutus. The Scotch grouse,
red grouse, or moor fowl (Lagopus Scoticus), is
reddish brown, and does not turn white in winter. The
white, or willow, ptarmigan (Lagopus albus) is found
in both Europe and America.
[1913 Webster]