Messenger hawk

Dragon Hawks are moderately sized birds of prey. In domestic use, they have canisters on their backs for transporting message scrolls. These hawks are used to deliver important messages throughout the Fire Nation and Earth Kingdom Colonies. Messages bear different colored ribbons depending on their importance and intended receiver. For instance, Dragon Hawks that carry messages tied with black ribbons are intended to be received by the Fire Lord and are usually of great importance.

Dragon Hawks seem to be the main tool in regular communication in the Fire Nation and can be bought at post offices, such as the one located in Fire Fountain City. Sokka bought a Dragon Hawk, Hawky, who became a temporary member of Team Avatar.

Anatomy
Dragon Hawks are moderately sized birds of prey with dark red or brown feathers all over their bodies and two long plumes falling from their tails. Their leg feathers are white.

Connection
The Dragon Hawks can be connected to messenger pigeons, especially homing pigeons. Homing pigeons were used to carry important military messages, much like the Dragon Hawk is in Avatar. Pigeons can only transport messages in a single direction, to one specific location, due to their homing instinct. However, Dragon Hawks appear to have the ability to locate their messages' intended destination even if they have never been there (as evidenced by Katara and Toph's dispatch of Hawky with a letter to Toph's parents).

Behavior
These birds have a calm temperament, are obedient, and are able to deliver messages to almost anywhere. They are very tame and loyal to their owners. They also seem to know that they have important jobs, and are regarded as more intelligent than other birds in the Fire Nation.

Trivia

 * Zuko originally had a Dragon Hawk as a pet in the pilot episode for Avatar: The Last Airbender. It was supposed to have a rivalry with Momo.