Birds carry their biggest, bulkiest muscles near the middle of the body to put weight at their center of gravity for efficient balance and flight efficiency . The largest leg muscles are on the thigh. Muscles of the distal wing and lower leg are small. Light meat does not have a good supply of blood rich in oxygen. These muscles can react quickly, but soon tire.
Dark meat is muscle with a greater blood supply and a supply of fat.
These muscles can work steadily for a long time without tiring. The dark meat of turkey thighs tells us turkeys are good runners and walkers.