carrier pigeon

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

• noun A homing pigeon, especially one trained to carry messages.
• noun Any of various breeds of large domestic pigeons having prominent wattles around the beak.

from The Century Dictionary.

• noun A breed of domesticated pigeon, having as its most apparent characteristic a huge wattle, like a miniature cauliflower, about the base of the stout beak, and a small wattle around the eye.
• noun A pigeon of a particular breed trained to convey from one place to another written messages tied to the neck or wing, or more commonly to the leg.

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

• noun A domestic pigeon which transports attached messages or very small parcels from the place where it is released to a familiar destination.

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

• noun a homing pigeon used to carry messages