What is it? Basically, a large number of the chickens’ feather fall out and new ones grow in, usually in autumn. Many of the old feathers are broken, ragged, and at the end of the laying season, lackluster. By molting, they have new ones that will protect them from the cold and–in nature–ensure they can fly adequately if a predator comes after them.
When hens molt, they also stop laying eggs so the energy of developing eggs goes into growing feathers. It’s like a rest for the hen from laying eggs for a short while. Like many things, the molt is triggered by the amount of light a bird receives. So, when the days grow shorter, its a signal to the bird body that its time to take a rest from laying and start growing new feathers.
Its always a shock to come out to the pen and see a huge pile of feathers lying on the ground or inside the coop. Often, my first thought is: which bird was eaten last night? But, happily, most often its the beginning of molt season.
I always hate the fact that I have to buy store eggs a few times a year, of course most often around Thanksgiving, but I figure its a price well-paid to let the hens have a rest and start back up when they are good and ready.