White meat is white because of the chicken's lack of exercise. White muscle is suitable only for short, ineffectual bursts of activity such as, for chickens, flying.
Dark meat, which avian myologists (bird muscle scientists) refer
to as "red muscle," is used for sustained activity, mainly
walking. The dark color comes from a chemical compound in the
muscle called myoglobin, which plays a key role in oxygen
transport. That's why the chicken's leg meat and thigh meat are
dark and its breast meat (primary flight muscles) is white.
Other birds, such as ducks and geese, have red muscle (dark meat)