a) Birds of a feather flock together.
That is, birds having the same feather type flock together. Is this the meaning of this idiom? Or could it have been
b) Feathers of a bird flock together.
which also means the same?
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Christine Ammer, The American Heritage Dictionary of Idioms, second edition (2013) doesn't consider "birds of a feather" an idiom—and consequently doesn't include an entry for it. However, Christine Ammer (same person), The Facts on File Dictionary of Clichés, second edition (2006) does consider it a cliché. Here is the entry from that dictionary:
birds of a feather Individuals of similar taste, background, or other characteristics in common. The term is a shortening of the proverb "Birds of a feather flock together," an observation made more than two thousand years ago by Ben Sira in the apocryphal book of Ecclesiasticus. The sentiment was transferred to human beings and repeated by numerous English writers from Shakespeare's time on.
With regard to the longer proverb, Martin Manser, The Facts on File Dictionary of Proverbs (2002) has this:
birds of a feather flock together People tend to associate with those of similar character, interests, or opinions; often used with derogatory implications: "Who are the guys that mostly get asked to the clubs? Preppies from St. Paul's, Mark's, Groton. It's kind of a common bond. You know, birds of a feather flocking together and so forth" (Erick Segal, The Class, 1985). The proverb dates from the 16th century or earlier in this form, but the sentiment it expresses is found in the Apocrypha "The birds will resort unto their like" (Ecclesiasticus 27: 9). It also occurs in a translation of Plato's dialogues by Benjamin Jowatt, published in 1871: "The old proverb says that 'birds of a feather flock together'; I suppose that equality of years inclines them to the same pleasures, and similarity begets friendship; yet you may have more than enough even of this." Some writers have substituted other verbs for flock, as in "birds of a feather laugh together (James Joyce, Ulysses, 1922) and "birds of a feather fight together [that is, on the same side]" (Leo Tolstoy, War and Peace, 1863–69).
Anyone who has watched flocks of starlings or pigeons swirling around a city sky knows that the expression accurately describes a real-world phenomenon: that many birds tend to congregate with others of their own species. As Robusto notes, the proverb uses feather in the sense of "plumage"—that is, "set of feathers." The American Heritage Dictionary of the English Language, fifth edition (2011) even dedicates a separate definition of feather to this particular figurative (or synecdochic) usage:
feather n. 1. One of the light, flat growths forming the plumage of birds, consisting of numerous slender, closely arranged parallel barbs forming a vane on either side of a horny, tapering, partly hollow shaft. 2. A feathery tuft or fringe of hair, as on the legs or tail of some dogs. 3. Character, kind, or nature: Birds of a feather flock together. ...
Nevertheless, the assertion that birds of a feather flock together is subject to overstatement, which is why birdwatchers are familiar with the term "mixed flocks." In fact, the highlands of Malaysia are known for a phenomenon called "bird waves," as explained in Montane Birds of Cameron Highlands (2014):
A bird wave or mixed foraging party is when birds of different species rush or fly through the forest together. This activity allows for insects and other preyto be disturbed/dislodged and fed on. Although it happens in the lowland jungles, the frequency in the highland jungles is so common that you might get to observe three events in the space of an hour.
What is amazing is seeing these vastly different species cooperate in such a manner—from tiny warblers to large barbets and fantails. You might actually get an opportunity to observe a less common bird that joins the activity.