It is important to note why Napoleon says the hens' eggs should be sold. He claims that the hens general make-up almost makes it impossible for them to contribute to the labor and construction of the windmill. But, they do have a commodity others value, and that could be their contribution: their eggs.
The hens stage a rebellion of their own which includes laying their eggs from the rafters so they break on the ground and are of no use. Napoleon responds by withholding their food. After five days, a few die. The rest decide its not worth it and give in to Napoleon's demands.