The breast meat is referred to as ‘white meat’. White meat is ‘white’ because of a lower
level of exercise for these muscles. The thigh and leg meat are referred to as ‘dark
meat.’ Dark meat is ‘dark’ because the muscles are used for sustained activity – in the
case of a chicken, chiefly walking. The dark color comes from a chemical compound in
the muscle called myoglobin, which plays a key role in oxygen transport. White muscle,
in contrast, is suitable only for short, ineffectual bursts of activity such as, for chickens,
flying. That's why the chicken's leg meat and thigh meat are dark and its breast meat
(which makes up the primary flight muscles) is white. Other species of poultry more
capable of flight (such as ducks, geese, and guinea fowl) have dark meat throughout.