What does the proverb "birds of a feather flock together" mean?

The proverb is giving an example of the way in which
things or creatures that are similar are attracted to each other. It is saying that the
same kinds of birds gather in groups with others of the same species - flocks of geese
or colonies of barn swallows, as examples. A flock would have all Canadian geese in it,
as opposed to being made up of some geese, some robins, some eagles, some hummingbirds,
and so on - all in the same closely bound grouping. The proverb applies to people, too -
humans tend to spend much of their time with others who share similar interests, are of
the same general age group, pursue common activities, and so
forth.