why do the hens rebel against napoleon? How do they rebel?

Napoleon was selling an enormous amount of eggs to the humans, which the hens weren't happy about. The hens rebel by laying their eggs in the rafters so that the eggs smash on the floor. Napoleon stops the hens’ rations and makes feeding a hen punishable by death. Nine hens die, supposedly of coccidiosis, during the five-day strike, after which the hens surrender.

why do the animals cease to sing "beasts of england"? what is the song that replaces ir, and how is it different from its predecessor?