Colorado State University
Colorado State: Pathophysiology of the Digestive System
A site on the disorders of the digestive system and on the similarities and differences between the digestion of birds and ruminants.
Other
Ornithology.com: Foraging and Nutrition
A very good discussion of the nutrition and feeding habits of birds.
Other
Ornithology: Metabolism and Thermoregulation
A good site that discusses the rate at which a bird's metabolism functions and its effect on the body temperature of the animal.