birds of a feather flock together.

Prov. Similar people tend to associate with each other. I always thought Amy was pretentious, and now she's going out with that snobbish boy, Louis. Birds of a feather flock together. George: Why do you think Donald is dishonest? Ned: All his friends are dishonest. Birds of a feather flock together.

McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.

be very wary when 'flocking' with certain 'birds'.