When a chicken is about a year old she will start to lose her feathers but don't panic, this is meant to happen. She is moulting. This is a completely harmless process of plumage rejuvenation.
Most fashion-conscious chickens will try to coincide this with the annual autumn Paris Fashion Week so that they know what next seasons must-have colour is going to be.
They will then spend between 4 and 6 weeks completing their new look. As with most women, even if you can't see any difference, you must compliment your chicken on how wonderful her new feather-style is. If you don't, she may well sulk and refuse to lay you any eggs.
Make sure the birds are well fed during this period as it takes a lot of energy to grow new feathers.
Because of all the energy taking up with moulting, your chickens will stop laying until their new feathers have grown. It is also important to remember not to clip your chickens wings when they are moulting.