mute swan

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A white swan (Cygnus olor) native to Europe and Asia and widely introduced elsewhere, having an orange bill with a black knob at the base and being the least vocal of the swans.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun a white swan, Cygnus olor, found throughout Europe and much of Asia, having a distinctive orange bill with a black tip; it is not actually mute, but is not very vocal.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun soundless Eurasian swan; commonly domesticated