The phrase ‘birds of a feather flock together’ is an expression, which generally means: people with similar tastes and interests stick together or form groups.  The phrase often stands on its own, is used as a response to something someone has said to you, or it is used to simply make an observation.  Here are some examples of how the phrase can be used.

Person 1: We hardly ever see my brother any more.  He is always hanging out with his artist friends.
Person 2: Birds of a feather flock together.
Person 1: That’s true they are all alike.

Person 1: Look at all of those teenage girls wearing almost exactly the same clothes.  They always do everything with one another too.
Person 2: Well, birds of a feather do flock together.

Sometimes people manipulate the words in this expression a little to fit a specific context or situation, but the meaning of the expression remains the same.  Here are some examples of what types of manipulations might be made to this phrase, while it still has the same meaning as above.

Person 1: Women of my age flock together; we are birds of a feather.

Person 1: I’m so happy Meredith and Aiden got married; they are birds of a feather.

In nature, birds of the same species do in fact frequently form flocks* and then fly together.   That is where this expression comes from!  Scientist who study this behavior explain that it provides birds ‘safety in numbers’ to be together, so that they are at less risk of predators.  The same general idea is true for people, as it is for bird.  Although people are less concerned with attacks from predators and more concerned with being around like-minded friends for comfort and conversation.