What did the hens do to rebel against the giving up their eggs.

The hens, rather than allow their eggs to be sold by Napoleon and his henchmen in chapter 7, lay their eggs on the rafters so that the eggs will smash on the floor of the barn.  They consider what Napoleon wants to do to be murder.  Because Napoleon and Whymper have agreed to sell 400 eggs a week, they punish the hens by not giving them any food.  The hens hold out for five days and then they give in, allowing Whymper to collect the 400 eggs a week.