any of a family (Chionidae) of white-plumed antarctic shorebirds distinguished by a horny, saddlelike sheath at the base of the upper bill
See sheathbill in American Heritage Dictionary 4
(shēthˈbĭlˌ)
noun
Either of two pigeonlike shore birds (Chionia alba or C. minor) of Antarctic regions, having white plumage and a horny covering on the base of the bill.