expression
birds of a feather flock together; like attracts like
Proverb meaning that similar people or things naturally group together. Literally 'cows with cows, horses with horses.'
あの二人はすぐに仲良くなった。牛は牛連れ馬は馬連れというからね。
Those two quickly became friends. Birds of a feather flock together, after all.
A traditional Japanese proverb. The exact origin is uncertain, but it likely arose from rural life, observing that cattle and horses naturally stay with their own kind.