Vague tip: Pay attention to the expression Erinna is making, also pay attention to the diary entries as it describes how Erinna is doing. She is an energetic predator, so even while asleep, she will be quick to react to an escape attempt if she's in a comfortable state.
For a more direct explanation and potential spoilers, you can read the in-game tip page for Harpy to understand. All predators and their somewhat hidden interactions are mentioned on their tip pages. Help > Predator Tips - Spoilers > find the predator you want info on.