What is the significance of the fly in" I heard a fly buzz", The bird in " A Bird came down the Walk", and the snake in " A Narrow Fellow

The fly, the bird, and the snake are simple images. These images represent Dickinson's preoccupation with nature. These images help Dickinson express herself on terms of her faith, immortality, death, and love.   

These lines reveal her relationship with nature's creatures:

Several of nature’s people

I know, and they know me;

I feel for them a transport

Of cordiality;