Muscles used for endurance, require more oxygen and therefore are full of blood vessels. Scientists call these muscles “slow twitch muscles”. Oxygen is carried by myoglobin, which gives the muscles their darker color. Slow twitch muscles contain myoglobin, which gives the meat a darker color.
Therefore, the muscles requiring more oxygen are darker.
The so-called fast twitch muscles do not require as much oxygen, as they are used for quick bursts of energy, tire quickly and are not used often.
These muscles use the muscle glycogen storage to release these sudden bursts of energy. Less oxygen used in these muscles requires less myoglobin, resulting in a lighter color of the uncooked meat.
Chickens, for example, although they are able to sporadically use their wings to “jump” a few meters at a time, are not capable of flying long distances.
Their breasts, therefore, have little need for a constant supply of oxygen and so are low in myoglobin making the breast meat appear white.
A chicken or turkey has both types of muscles: dark in the thighs (continuously used for walking) and white in the breast (rarely used).
Ducks and geese, however, have a different lifestyle. These birds are considered as poultry by the meat industry (i.e. white meat in the red/white meat category).
However, they fly for long periods of time and their breasts require more oxygen and myoglobin.