Molting is a natural process that chickens go through yearly to renew their feathers. When molting, chickens gradually lose their old feathers and replace them by growing new features. Molting can take anywhere from two to six months to complete. When a hen is molting, she usually will stop laying.
Molting can also result from stress, which can occur if a hen has to go without feed or water.
For more information about molting, see “Molting of laying hens” from the Mississippi State University Extension Service.