‘Of the same feather‘ means ‘of the like plumage‘! Birds of same kind naturally flock together. Peacock flock with peacocks, and sparrow with sparrows. Lambs don’t make friends with wolves, nor mice with cats. So it is with men, too. Persons of like tastes and dispositions go together. The rich rarely mix with poor, the good find no pleasure in the company of the bad, the idle don’t make friends with the hard working. Fools do not like company of wise men and vise versa. Sinners feel out of their element in presence of saints, and saints shun the company of the sinners.
Hence, it is rightly said, ‘Birds of the same feather flock together‘.