Metamorphoses Book 14: The Island of Circe
Macareus told how he had been one of the men Circe transformed to a swine with a magic drink while they were on her island. He was only changed back to human form because Ulysses married Circe and asked for his men's recovery as his wedding gift. The crew stayed on the island for a year, and that's how Macareus learned the story of Picus.