hi
its a fungal problem. i had it afew years ago and tryed everything and i mean everything.a definate cure is to use vircon s mix 50grams to a gallon of warm water in a garden spray spray all birds young, old birds, and stock. around the chest area and back of neck right up to head dont rinse give them a bath afew days later in future use it in bath water on fine day also spray all loft and nest bowls and every thing else. you can get it out of bhw, or rp from pigeon pharmacy. i promise you it will cure it.