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.
National Wildlife Federation
National Wildlife Federation: Bird Feeding 101
Next to gardening, feeding wild birds is the second most popular hobby in North America. This concise resource describes how to provide a safe and healthy feeding environment for the birds visiting your backyard.
Curated OER
Ornithology.com: Foraging and Nutrition
A very good discussion of the nutrition and feeding habits of birds.