feather 当然是可数名词。词典释义为：

【可数名词】A bird's feathers are the soft covering on its body. Each feather consists of a lot of smooth hairs on each side of a thin stiff centre. 羽毛

Birds of a feather flock together.（谚语：物以类聚，人以群分。）

不定冠词a 有一特殊用法，表示“同一（种；类；个）”，意思等于the same。例如：

They are of an age. = They are of the same  age.

他们同岁。

a feather 在这里是指"同一类羽毛", 不是"一片羽毛"。

Birds of a feather flock together.

这句话字面是："有同一类羽毛的鸟聚结在一起", 意译则是"物以类聚"。

它的另一种说法：Things of one kind come together.

该谚语就是这样来的。