Ratbirds were small, flying, rodent-like birds related to snickets. They nested in many places, including sky ships. On sky ships, ratbirds picked off other sky vermin and protected the ship's cargo. According to legend, ratbirds only left a sky ship that was truly doomed, though evidence from the books suggested that they simply left when a ship was threatened or not skyworthy. Ratbirds were also excellent navigators, capable of finding their way back to their nest no matter how far away they were.
For this reason, they could be used to deliver messages, because a ratbird that nested on a sky ship could always find its way back to that ship. However, some villainous individuals would use ratbirds to send Messages of No Return.
Ratbirds could also be used for reconnaissance instead of just allowing for point-to-point communication. Returning ratbirds would dance to convey information before entering their cages. The dips, dives, and somersaults in their "wiggle-dance" were capable of telling a sky ship's ratbird-keeper the direction and distance of any object. During the events of Cloud Wolf, Steg Jambles used the intelligence gathered from a ratbird's dance to locate the Great Sky Whale.