How do the animals assemble themselves?

From the text:

First came the three dogs, Bluebell, Jessie, and Pincher, and then the pigs, who settled down in the straw immediately in front of the platform. The hens perched themselves on the window-sills, the pigeons fluttered up to the rafters, the sheep and cows lay down behind the pigs and began to chew the cud. The two cart-horses, Boxer and Clover, came in together, walking very slowly and setting down their vast hairy hoofs with great care lest there should be some small animal concealed in the straw.

The two horses had just lain down when a brood of ducklings, which had lost their mother, filed into the barn, cheeping feebly and wandering from side to side to find some place where they would not be trodden on. Clover made a sort of wall round them with her great foreleg, and the ducklings nestled down inside it and promptly fell asleep. At the last moment Mollie, the foolish, pretty white mare who drew Mr. Jones's trap, came mincing daintily in, chewing at a lump of sugar. She took a place near the front and began flirting her white mane, hoping to draw attention to the red ribbons it was plaited with. Last of all came the cat, who looked round, as usual, for the warmest place, and finally squeezed herself in between Boxer and Clover; there she purred contentedly throughout Major's speech without listening to a word of what he was saying.