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. The coloring and the sound are different, however. Tweety is supposed to be a canary.
Canaries are small yellow birds very similar to parakeets.
They sing often and can even be taught to speak. Donald Duck is a common domestic duck since he is white with an orange bill just like any domestic duck you might see at the park. As for Daffy, while there is no duck that is truly black, there is a black duck. The American Black Duck, a very common duck similar to a female Mallard, lives throughout Virginia and in other parts of the country. It is a very dark brown and sometimes appears to be black, especially if it's wet or the light is bad.