Holly Faulkner, Project Assistant says:

Hi Darla, Bald heads are more common in cardinals and Blue Jays, as they sometimes molt all of their head feathers at once. You can read more about this here. However, Northern Cardinals have black skin, and a different color skin on the head of a bald-headed cardinal would indicate an ailment, such as feather mites, lice, or an environmental or nutritional factor. In this case there’s not a lot that can be done. It is likely not caused by a disease, however we always recommend regularly cleaning your feeders to help prevent the spread of disease, even if you don’t see sick birds. You can learn more about the best practices in cleaning your feeders here.