From Animorphs book 49: The Diversion, by K.A. Applegate.
The Yeerks finally realize the Andalite bandits are humans. Tom captures Jake's parents, turning them into Controllers. They return home for Jake, who is expecting to take his family to refuge. Jake's mother fires a Dracon beam at him, and a black scar rips across their house. Jake morphs falcon as they approach, revealing himself as an Animorph to give his family hope and their Yeerk captors warning.