Birds of a Feather

The idiom Birds of a Feather is from the proverb "Birds of a feather flock together," meaning that similar people congregate. In old poetic English, "birds of a feather" means birds which have the same kind of feathers, so the proverb refers to the fact that birds congregate with birds of their own species. This proverb has been in use since the 16th century.

Plato used the phrase in The Republic, 360 B.C.E, "I will tell you, Socrates, he said, what my own feeling is. Men of my age flock together; we are birds of a feather, as the old proverb says;"

http://en.wikipedia.org/wiki/Birds_of_a_Feather_(disambiguation)