"Members of an avian species of identical plumage congregate" is likely a rephrased form of the common proverb, "Birds of a feather flock together." Both phrases imply that people tend to associate with other people who have similar interests and characteristics as themselves.
"Birds of a feather flock together" is an idiomatic expression, but it is literally accurate as well. Birds are most likely found in groups of the same species, just as people are usually to be found associating with others that are most like themselves.
The origin of the phrase is believed to be Old English, originally written by the scholar William Turner in the year 1545.