Avisa is frequently compared to an Eagle that flies high above all sin. We must remember that the book is written in Ironia ie the book is saying one thing but meaning the opposite. What the author is really saying is that Avisa is an eagle who stoops to satisfy her carnal lust. The image of a stooping Eagle is associated bibically with the fall of Adam.
The Elizabethan birds of Chastity were swan or dove or turtle dove. Avisa is never called any of these.