Sources: PowerPoint, Web, text (p. 365-458)

1. As the line of dinosaurs that became birds became endothermic, insulation was selected for. Longer scales provide more insulation, up to a point. At the point scales were as long as they were going to get to increase insulation, they provided enough lift to allow the pre-birds to hop higher or glide better (to catch prey or avoid predators). This selected for even longer and more modified scales that would both provide lift and insulation.

2.

Huge flight muscles, that require:

• Rigid skeleton: fused bones, uncinate processes on the ribs, wishbone.
• Efficient Respiration: one way flow respiration
• Hot Body Temperature: speeds nerve and muscle action

Weight Reduction

• Hollow bones
• Lack teeth
• One ovary (instead of two)
• No urinary bladder
• Air Sacs

3. Our lungs are dead end sacs filled with stale, unmoving air.
Bird lungs have fresh air moving one-way through them (as the air flows from posterior air sacs through the lungs and into the anterior air sacs) which not only provides fresh air (vs stale air) but also allows countercurrent gas exchange (air moves opposite direction of blood flow, allowing complete diffusion of oxygen/carbon dioxide into and out of the blood).