In general, what does Christie compare each of the characters to at the beginning of chapter 13? Why does she do this?

At the beginning of Chapter Thirteen, all of the guests are to compared to wild animals, or in Vera's case, a bird. These animals remind us of prey, which is exactly what the people they describe are..... they are in danger, and they are hunted. 

Like a wary old tortoise, Mr. Justice Wargrave sat hunched up, his body motionless, his eyes keen and alert. Ex-Inspector Blore looked coarser and clumsier in build. His walk was that of a slow padding animal. His eyes were bloodshot. There was a look of mingled ferocity and stupidity about him. He was like a beast at bay ready to charge its pursuers. Philip Lombard's senses seemed heightened, rather than diminished. His ears reacted to the slightest sound. His step was lighter andquicker, his body was lithe and graceful. And he smiled often, his lips curling back from his long white teeth.

Vera Claythorne was very quiet. She sat most of the time huddled in a chair. Her eyes stared ahead ofher into space. She looked dazed. She was like a bird that has dashed its head against glass and thathas been picked up by a human hand. It crouches there, terrified, unable to move, hoping to save itself by its immobility.