Ovid Book 6

Tereus, the tyrant from Thrace, enters the narrative. He liberates Athens from barbarians and marries Procne, the daughter of the king of Athens, Pandion. The marriage is ill-fated. Juno, Hymenaeus, and the Graces do not attend the wedding. After five years of marriage, Procne asks Tereus for permission to see her sister, Philomela. Tereus sets sail for Athens to fetch Philomela. As soon as he sees Philomela, lust grips him. 

Back in Thrace, he repeatedly rapes her and hacks off her tongue to prohibit her from speaking. Philomela weaves a portrait of Tereus’s crime onto cloth and sends it to Procne. To get revenge, Procne slays Itys, her only child with Tereus, and serves him to Tereus as a meal. Procne and Philomela tell Tereus that he has eaten his son, and Tereus goes mad. He wants to kill the sisters, but they escape by turning into birds. Tereus becomes a bird as well. 

These transformations occur quite often in Ovid's work. It appears as though birds are the transformation for escaping certain death. Tereus is the embodiment of everything evil when considering his actions.