You do not need to cook mushrooms before serving them to your chickens. You do however need to cut them into small chunks for your chickens to swallow. That being said, chickens are not generally fond of the rubbery texture of raw mushrooms and are unlikely to consume them. Instead, it is better to fry mushrooms in water (never oil/butter), grill, or boil them in water. This will make them easier to eat and more enticing.