the problem is this. we have 4 hens all hybrids. they have been perfectly healthy and we get 4 eggs a day. all four hens come from the same farm that was spotless. purchased about 3 months ago. A few days ago we noticed suddenly one of the hens had a completely bald area around the vent and tummy. today we have noticed another hen has lost feathers in the same are and on her legs too and another starting to loose feathers around the vent. the cage is regularly cleaned and kept clean. the birds have been treated against red mite and we are using anti peck spray in case they are attacking each other however we have seen no evidence of this and they all seem to get on well together.
im looking for ideas as to what this could be. it seems strange that it is only one area that is affected and their heads, wings and backs are in perfect condition
Since you already checked and treated for bugs, that eliminates one possible cause.
Here are some others possible causes.
Over crowding in the coop. Over crowding causes stress in birds that can lead to feather picking. So can being bored. If you keep your birds inside, try giving them a pan with dirt for a dust bath. Give them perchs to climb, boxes to explore. Provide a treat they have to peck at.
Feather picking can be a sign the birds have a protein problem. Chickens will eat blood rich feathers to get protein from. Check what kind of feed you are giving them? You said your girls are all laying? Are you feeding them layer rations? If not, try switching them to layer which is higher in protien. You can also help by giving your girls extra protein as a treat, like tin catfood made from fish, cooked hamburger or mashed up cooked eggs. I use catfood and boiled eggs mashed up. Chickens love it!
If all that fails and the birds are still plucking each other, you may have to seperate them. I had that problem once with a couple of hens whom when together would select a bird in the coop and proceed to peck the stuffing out of them. They balded the poor roo I had with them. Ended up I had to seperate them and put them with other flocks. The problem then stopped.