birds of a feather flock together; like attracts like
Proverb meaning people of similar character or background naturally gather. Often used as the second half of the longer saying 牛は牛連れ馬は馬連れ.
See also: 牛は牛連れ馬は馬連れ
There is a proverb: 'Cows go with cows, horses go with horses.'
彼の友達を見れば、彼がどんな人かわかる。馬は馬連れというからね。
You can tell what kind of person he is by looking at his friends. Birds of a feather flock together, after all.
A traditional Japanese proverb. The exact origin is uncertain, but it reflects the observation that animals of the same kind stay together, applied to human relationships.