I don't exactly know what "the shapes" mean in the sentece.
Darwin realized that change in nature is very slow. But, he argued, we know that change can be much quicker when human beings are in charge of the process, selecting the traits they desire in their plants and animals. He called this artificial selection, and humans have been doing it for thousands of years. Darwin bred pigeons, and exchanged many letters with his fellow pigeon fanciers. He knew just how quickly the shapes and behaviour of their show pigeons could change, when the breeders carefully selected pigeons with certain traits for breeding chicks.
I don't exactly know what "the shapes" mean in the sentece, "the shapes and behaviour of their show pigeons"
Could you help me?