Heat butter in kettle and saute pheasant pieces until brown. Remove bird and add onions, garlic, celery, and carrots. Saute until tender. Add stock and return pheasant to kettle. Simmer 10-15 minutes. Add mushrooms corn and thyme, season with salt and pepper, and simmer until meat is tender. Remove pheasant pieces, pull meat from bones, cut up meat, and return to kettle. Reheat.