From Simon Ings: why is chicken supreme? And what is it with eating only part of a chicken anyway?
You raise an interesting question, Simon, and it goes back to the Chicken Wars of 13th century Portugal. The Peri-Peri sect of the Catholic Church strongly favoured the full bird approach, considering the integrity of the fowl to be a key element in its consumption, giblets and all. The breast and wings were thought to be religiously significant, and to separate them would be sacrilege.
A rival faction, however, known colloquially as the 'Nuggetters' prefered to segment the bird and serve it in individually blessed pieces (au vin), and it was during the Convocation of 1287 that the Papal Decree was issued declaring the individual chicken breast the supreme aspect of the bird, and recommending a cheese sauce.
It was only in the 18th century that the Ukrainians got involved with their Orthodox Kiev approach. But that's another story.