Why is some bird meat white and some dark?
It's a matter of exercise. Leg and thigh meat
is dark because that's what chickens use for whatever sustained exercise they happen to get in those tiny pens.
The darkness comes from myoglobin, which helps move oxygen around.
White meat, on the other hand, is found in the breast, whose muscles birds use for short bursts of activity, such as flying. Ducks and geese, who are better flyers (and who don't live in tiny pens), have dark meat all over.
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What would you attempt to do, If you knew you couldn't fail?