feather就是鸟身上的羽毛，bird就是鸟，birds of a feather flock together跟中文里说的：物以类聚，人以群分的意思很相似。

【例句】
Just as a saying goes," Birds of a feather flock together."
正如谚语所说，“物以类聚，人以群分。”

Birds of a feather flock together, and therefore they are such good friends.
物以类聚，人以群分，所以他们能成为好朋友。