These are probably the most famous. In a sense, these birds ARE real. They are all based on birds that exist somewhere in the world. Even the behaviors of some of these cartoon birds are based on the real things.
Cartoonists often study an animal that they plan to turn into a cartoon.
Then, they take one or more of those characteristics and exaggerate them make them bigger, in a way, than they really are. Take the Roadrunner, for example. They don't go so fast that their legs look like circles. Well, the way it is drawn does look somewhat like a real Greater Roadrunner which does have a crest and a somewhat long neck. These are exaggerated in the cartoon.