Hi, I have a hen that has a recurring problem. On her right foot, she keeps developing a "stone" on the webbing between her toes. I can squeeze it and a small "stone" pops out and leaves a hole in the webbing. I put an antibiotic ointment on it and haven't had any problems with infection. It's like popping a blackhead! Has anyone else had this problem, and if so, do you what it is and what causes it. Thanks!