In "And Then There Were None", how is Vera Claythorne like a bird?

In And Then There Were None by Agatha Christie, she keeps referring to the characters like certain animals. I understand how Phillip Lombard is like a panther and Wargrave is like a tortoise, but I don't get how Vera is like a bird. Aren't birds supposed to be fluttery and flighty? That's not at all like Vera. Could someone help me understand this? How is she like a bird? I need it for an essay but I'm not grasping Vera Claythorne as a bird at all.