Birds of a Feather Flock Together
Those with similar interests or of the same kind tend to form groups.
A: John and James seem to get along really well
B: Well, birds of a feather flock together. They both love reading comics.
Did you know?
In nature, birds of the same species often flock together as a safety precaution, since congregating in large groups would reduce their risk of predatation. This behavior of birds is where this idiom originates from.