Heracles and the Stymphalian birds. Detail of a Roman mosaic from Llíria, Spain.

The Stymphalian birds are man-eating birds with beaks of bronze, sharp metallic feathers they could launch at their victims, and poisonous dung.

"These fly against those who come to hunt them, wounding and killing them with their beaks. All armour of bronze or iron that men wear is pierced by the birds; but if they weave a garment of thick cork, the beaks of the Stymphalian birds are caught in the cork garment, just as the wings of small birds stick in bird-lime. These birds are of the size of a crane, and are like the ibis, but their beaks are more powerful, and not crooked like that of the ibis." — Pausanias. Description of Greece, 8.22.5

These birds were pets of Artemis, the goddess of the hunt; or had been brought up by Ares, the god of war. They migrated to a marsh in Arcadia to escape a pack of wolves. There they bred quickly and swarmed over the countryside, destroying crops, fruit trees, and townspeople.

The Sixth Labour of Heracles. The Stymphalian birds were defeated by Heracles (Hercules) in his sixth labour for Eurystheus. Heracles could not go into the marsh to reach the nests of the birds, as the ground would not support his weight. Athena, noticing the hero's plight, gave Heracles a rattle called krotala, which Hephaestus had made especially for the occasion. Heracles shook the krotala (similar to castanets) on a certain mountain that overhung the lake and thus frightening the birds into the air. Heracles then shot many of them with feathered arrows tipped with poisonous blood from the slain Hydra. The rest flew far away, never to plague Arcadia again. Heracles brought some of the slain birds to Eurystheus as proof of his success.

The surviving birds made a new home on an island of Aretias in the Euxine Sea. The Argonauts later encountered them there.

According to Mnaseas, they were not birds, but women and daughters of Stymphalus and Ornis, and were killed by Heracles because they did not receive him hospitably. In the temple of the Stymphalian Artemis, however, they were represented as birds, and behind the temple there were white marble statues of maidens with birds' feet.