The bantam Roster become bald

My neighbor bought a couple of beautiful white banyak chicken. The Roster looks very good and he often crow every morning. The cople live at an iron rounding cage.
I am hubdred percent sure that the roosters was bot hald. He has complete neck feather. Recently, I have seen the feather losses. Fortunately, the hen does not suffer feather losses. Perhasp the carin g of the chicken is veru bad. I often see the manure below the cage. Rarely the owner clean the cage.
Perhaps the chicken food also influencebthe necke feather. I suspect that the chicken that has bad chicken food. Perhaps you can tell me why the chicken become bald?