Whenever you see, hear, or taste something, you can only tell so much about it based on what you sensed. This often isn't enough, so we make assumptions about what the rest of the thing is like — when people hear "bird", they probably think of something that is fist-sized, flies, and sings, even though none of these things are true about every single bird. See prototype theory
When a set of such assumptions about something become "common knowledge," they form a stereotype. Most of the time, nobody notices, as in the case of birds. This is very useful when you're writing fiction, because it lets you save a lot of space and time that you'd otherwise have to spend describing something in detail, when it isn't really important to your story.
is a stereotype that writers find useful in communicating with readers. Some stereotypes that originally developed outside of fiction lend themselves readily to use as tropes, and some tropes turn into stereotypes outside of fiction. Some such tropes are: