The Collaborative International Dictionary of English v.0.48:

Sheathbill \Sheath"bill`\, n. (Zool.)
   Either one of two species of birds composing the genus
   Chionis, and family Chionidae, native of the islands of
   the Antarctic seas.
   [1913 Webster]

   Note: They are related to the gulls and the plovers, but more
         nearly to the latter. The base of the bill is covered
         with a saddle-shaped horny sheath, and the toes are
         only slightly webbed. The plumage of both species is
         white.
         [1913 Webster]