There are different schools of thought when it comes to feeding falconry birds. Some Europeans feed meat based on its nutritional value to control how hungry the bird is. They feed additional roughage, such as fur, so the bird can digest properly. Some Americans feed their birds whole food such as mouse or quail, reducing the need for supplements and additional roughage. All birds of prey used in falconry eat a strictly carnivorous diet.
In all cases, a bird's diet is carefully measured to control its weight. Weight determines how hungry the bird is and how lazy it will act. A bird that is "fat," or has a higher weight, will be more likely to fly away or not hunt. A bird that is somewhat underweight will act aggressively, and a bird that is severely underweight will have health problems.