This appears to be caused by parasites. Whether they are feather lice or something else I don't know. There are always a few Cardinals like this around from mid-July until they under go their per-basic molt. The skin of Cardinals is usually lighter and the dark color must be from exposure to the sun. Further, I don't know if the birds remove the feathers by scratching or if the irritation from the parasites cause the feathers to molt prematurely. My two cents, Wilbur Fred. Co.