In the Twelve Labors of Hercules, the Stymphalian Birds were associated with Lake Stymphalus.
Lake Stymphalus lies in Arcadia in the Peloponnese. In mythology, its marshy surroundings became the home of aggressive birds with metallic feathers and, in some accounts, sharp bronze beaks or claws. Hercules was sent to drive them away during his sixth labor.
The birds were difficult to reach because the marsh could not easily support a person. Athena helped Hercules by providing a device that made a loud noise, allowing him to frighten the birds into the air. Hercules then shot many of them with arrows.
Lake Lerna belongs to the Hydra story, not the Stymphalian Birds. Keeping the two watery settings separate prevents one of the most common location mix-ups in the labor cycle.