By definition, round characters are well-developed and flat characters are not developed.  Sometimes round characters are also considered dynamic characters (as compared to static) because they have the ability to change and grow.  Chanticleer is considered a round character because we actually learn quite a bit about his life and personality through the this tale.  Chanticleer is a very busy with all of "lady" hens and appears to be quite popular.  He is also characterizedto be lacking in courage as he reveals his bad dreams to Pertelote, his favorite of the hens.  She and he have an interesting relationship because she challenges his fears and calls him out on his lack of bravery.  An interesting contrast to this side of his personality is very strong pride over this singing voice.  It is this pride that gets him trouble with the sly Russell the Fox.  Luckily, he is able to escape the Fox with a clever reversal, but all of this story reveals several interesting sides of Chanticleer, making him more than a stereotypical rooster who struts around and makes a lot of noise. He learns a lesson about his fears and his talents and is a changed rooster by the end of the tale.

A round character is defined as a character that is neither purely good nor purely evil. A round character should have both flaws and virtues that are evident.

A the beginning of the tale, Chanticleer is praised for his attractiveness and excellent crowing abilities. These would both be positive qualities. Chanticleer's worry in his dream also indicates that he is able to feel, and to have concern. Chanticleer also expresses love towards Pertelote.

Chanticleer's flaw is his vanity. He is wooed by the fox's flattery enough to be captured. He loves to be flattered. He learned his lesson by the end of the tale, though, when he smartly escapes the fox's jaws.

What are some details from "The Nun's Priest's Tale" of The Canterbury Tales that support the claim that Chanticleer is a round character?

Round characters, characters who are fully developed, have many sides to their personalities, so much so that they are often perceived by readers as realistic. The Nun's Priest's tale is much like a fable as the rooster, the hen, and the fox are anthropomorphized, or given human qualities, and the tale leads to a moral lesson.

Chanticleer, the rooster, is a vain male that has several hens, the favorite of which is "the winsome Pertelot." One day

This Chanticleer he groaned within his throatLike man that in his dreams is troubled sore.

 Pertelot asks what is troubling him, and he replies,

Ah, madame,I pray you that you take it not in grief,By God. I dreamed I’d come to such mischief....

And, he tells his love about a beast that kills him. Pertelot scolds him for not being a man, though he has "a beard."  In reply to her, Chanticleer relates the story of a man who had a similar dream and was, in fact, killed.  But, foolishly he listens to Pertelot--"took the counsel of his wife, with sorrow"--and does not heed his dream.  Later, however, a fox...