In Greek mythology, which bird was sent to feed on Prometheus' liver?

In Greek mythology, Prometheus (possibly meaning "forethought") is a Titan, culture hero, and trickster figure who is credited with the creation of man from clay, and who defies the gods by stealing fire and giving it to humanity, an act that enabled progress and civilization. Prometheus is known for his intelligence and as a champion of mankind and also seen as the author of the human arts and sciences generally. He is sometimes presented as the father of Deucalion, the hero of the Greek flood story.

The punishment of Prometheus as a consequence of the theft is a major theme of his mythology, and is a popular subject of both ancient and modern art. Zeus, king of the Olympian gods, sentenced the Titan to eternal torment for his transgression. The immortal Prometheus was bound to a rock, where each day an eagle, the emblem of Zeus, was sent to feed on his liver, which would then grow back overnight to be eaten again the next day. (In ancient Greece, the liver was often thought to be the seat of human emotions.) Prometheus is freed at last by the hero Heracles (Hercules).