Board Thread:Questions and Answers/@comment-27612062-20160118002057/@comment-5014364-20160208183051

Ravens are trained to return to the castle they were raised at. Like carrier pigeons. Maesters then have to transport them by land in cages; i.e. ravens trained to return to King's Landing have to be transported by land to Winterfell.

They've been doing this for thousands of years and Maesters transporting ravens in carts by land are usually protected; they've been operating the raven network for generations so it functions pretty smoothly.

Once in a great while there are a few prized ravens that can learn to fly back and forth between two locations. VERY rarely, ravens can be trained to travel between 3 or 4 castles, but such birds only occur once every century or so.

Even in real life, ravens are actually quite intelligent animals.

They don't have a regular "mail" system of letter-carriers. Short messages are sent by ravens. Longer or more important charters, formal letters, and so forth are sent by specifically sent messengers, etc. -- i.e. Robb sends his declaration of independence to Cersei at King's Landing via a messenger under flag of truce.