Birds of a feather flock together

People of the same (usually, unscrupulous) character associate with one another, as do birds of the same species. Cf. [Apocrypha Ecclesiasticus xxvii. 9] (AV) The birds will resort unto their like, so will truth return unto them that practise in her.

Byrdes of on kynde and color flok and flye allwayes together.
[1545 W. Turner Rescuing of Romish Fox B8]

Birdes of a feather will flocke together.
[1599 J. Minsheu Spanish Grammar 83]

Our English Proverb‥That birds of a feather will flock together. To be too intimate with sinners, is to intimate that you are sinners.
[1660 W. Secker Nonsuch Professor 81]

It is literally true in the systematised roguery of London, that ‘birds of a feather flock together.’
[1828 Bulwer-Lytton Pelham III. xv.]

Birds of a feather flock together, so the second thing you should do is find another friend who's less troubled than the first.
[2001 Washington Times 15 July D7]

Related to: associates; human nature; similarity and dissimilarity

Bibliography of major proverb collections and works cited from modern editions is available here.

Individuals of like character, taste, or background (tend to stay together), as in The members of the club had no trouble selecting their yearly outing--they're all birds of a feather. The idea of like seeks like dates from ancient Greek times, and "Birds dwell with their kind" was quoted in the apocryphal book of Ecclesiasticus. The full saying in English, Birds of a feather flock together, was first recorded in 1545.