PERSPEX DOORS ON NESTING BOXES: this has saved many a chick from ´down plucking' and ´feather plucking'. It is imperative that when the perspex door is fitted that you remain in the breeding room until the hen and/or cock have been encouraged back into the nesting box. The sudden increase in light tends to frighten the cock and hen away from the box so it is imperative you check they have fed their chicks to avoid the death of one or more chicks from starvation.
Once the cock and hen have adapted to the light the hen will generally round up the young and keep them at the back of the box. In many cases she has been known to totally clean the bran/nesting material out of the nesting box, beak full by beak full -maybe when she has completed this task she is too tired to pluck the chicks! Just remember to replace the bran daily so she has something to keep her occupied.
If possible leave a small gap between the perspex and top of the nesting box, or drill holes in the perspex or side of the nesting box, so air can circulate in the box - the box can become quite humid if we experience a hot day. I have had a wonderful success rate using this method.
Make a note in your records, against the hen or cock, so you can use this method again if it has been previously successful.