His hunch is correct. She is capable of speech (indeed, she misses using it a lot) but Companions such as herself are not supposed to be able to speak. If she does, she will reveal herself to be an illegal mutation which is a crime punishable by death. It's safer to stay mute.

When he asks her that, she freezes. No, she's letting too much show again. A slow shake of her head. Nothing. It's nothing. Yet, she isn't doing herself any favors by pulling one wing around in front of her to hide behind bashfully. When she peeks back out again to look at him with big blue eyes and slowly raises a hand to point to her mouth, she hopes it conveys that she's hungry and it's okay. She'll eat whatever she can.