Its usually over within 6 weeks
Is there a cockerel in with them that could explain the bare patches on the back. Some birds pace their runs and rub against the wire often causing bald patches at the front
If both of these and the usual suspects of mite and lice can be disregarded then they are probably just moulting as you say and the feathers will grow back eventually. if they are still laying this wont help some some extra protein in the form of chick crumb, pigeon food or tinned fish will be appreciated by them
