Ravens

Ravens are a species of large, black-feathered birds native to Westeros. They are uncommonly intelligent animals and can be trained to serve as messengers. They can fly great distances at speed, and are used by the maesters of Westeros to pass messages between the castles and cities of Westeros. Through complx training methods, the maesters are able to send messages from almost any castle in Westeros to any other.

Although most messages brought by ravens are standard and innocuous, they are also linked to ominous tidings, giving rise to the phrase, "Dark wings, dark words."

In the books
In the Song of Ice and Fire novels, ravens are depicted as being identical in appearance but somewhat smarter than 'real' ravens. The ravens of Westeros can replicate simple human speech in the manner of parrots. There is also a cleverer but much rarer species of white raven, which the maesters dispatch only when they agree that the season has turned.