I really hope that Mareks is not the cause either. Tumors can be common in chickens, and Mareks is not always the cause. If something happens to her, I would check into getting a necropsy done by your state vet, and have them test for Mareks. Here is a link for contact info, and they list $30 as the cost of a necropsy (although testing could run more): http://www.freshfromflorida.com/Divisions-Offices/Animal-Industry