Birds of a feather flock together means that people who have many similarities intend to hang out together.
birds of same feather flock together
It means people and animals tend to stick to their own kind.
Because they are limited by the collective intelligence of only their flock.
"Birds of a feather", etc.. is a simple way of saying that like draws like. There is comfort in recognition, so we tend to choose our companions by how much alike we are. The opposite, "opposites...