Athlete's foot ointment or women's vaginal yeast cream will do. The way we work things here is to suggest things to treat what appears to be a likely issue since it's impossible to diagnose from the internet. By trying a treatment and seeing if it makes a difference, you will be ruling out that...
What is your location and climate? Are you in a sub tropical region? Has it been warm and wet?
My first guess is that your chickens are developing a fungus called favus. It can be treated with an anti-fungal lotion or ointment.