Bird bones, as evidenced by the absurdly low body weights and many broken bones every time they touch the ground.

I've always suspected that the pros have very little in the way of anaerobic metabolism. Being nearly 100% dependent on the anaerobic side myself, I sweat like a pig, even if it's cold. Most everyone I've known who is exceptional at endurance stuff seems to hardly sweat. Maybe aerobic energy produces less heat? Sweat gene theory would fail if everyone produces the same amount of heat from the same amount of energy output. You have to keep cool somehow, and I don't see any other heat dissipation mechanism that is unique to birds, err pros.