Earth Life
Earth Life: A Bird's Heart and Blood
A very good site about the circulatory system of a bird. It is reader friendly, and includes a fair amount of information. Good diagram of the circulatory system.
University of Illinois
University of Illinois Urbana Champaign: The Heart of the Matter
A good quick look at the comparative anatomy of the heart of a typical bird and the human heart. Gives some of the facts behind the differences.