carrier pigeon 
noun
A pigeon trained to carry messages tied to its neck or leg.When I fell sick, I sent a carrier pigeon down to the nearby village of Kishon....
- Sending a carrier pigeon down Pennsylvania Avenue with a diplomatic note doesn't work in the United States.
- The biggest shortcoming of the book is its paucity of illustrations: we're told over and over about amazing creatures from the gomphothere to the carrier pigeon, but only a handful of them are pictured.