The Stymphalian birds were birds sacred to the God of War, Ares. They had beaks made of bronze, feathers made of metal, and their dung was toxic. They lived in a swamp into which no man could walk. For his sixth labor, Heracles was told he had to kill them, or drive them away so they would not bother people again.
The Goddess Athena came to heracles help in this labor: seeing that Heracles could not get close to the birds, she gave him a special rattle made by a great forger (metal-worker) named Hephaistos. The rattle, called a krotal, frightened the birds into the air where Heracles shot some of them down with his poisoned arrows, whilst the others flew away (they were found in a later Greek story).