What is the the historical background of the story of the "Nun's Preist's Tale"? Why did he use animals instead of humans? What is the moral that story discuses?
In this format (the fable), beasts personify humans and exaggerate Man's characteristics, usually for the purpose of teaching a lesson. The characters, as in this case with Chanticleer, often make use of classical learning to solidify their moral instruction.
Chaucer probably based this story on the French Roman de Renart (Reynard is a character in the "Old French Le Roman de Renart" written by Perrout de Saint Cloude around 1175) and the German Reinhart Fuchs (the oldest German beast-epic that we possess; The date of its composition is about 1180); but, as was his custom, the author of The Canterbury Tales dramatically altered the plots. In the European models, the rooster is a self-centered idiot who repeatedly refused to listen to warnings. As the reader has observed, Chaucer's Chanticleer, although somewhat vain, is a victim of love. He overrides his own better judgment and goes into the yard to please Partlet whom he loves very dearly. It is, therefore, for love of Partlet that Chanticleer becomes the fox's victim.
The obvious moral lesson of the foolishness of succumbing to empty flattery diverts attention from a more subtle warning to beware the advice of women. This was a popular medieval theme. Woman, the original seductress, was the source of much of Man's sinfulness. As the weaker and less intellectually endowed of the two sexes, Woman was not a reliable counselor.