…aside from the looks of course.
When it comes to birds with clear white meat/dark meat distinctions, they have one thing in common – they’re flightless birds. Chickens and turkeys use their wings for balance, quick escapes and not much more. The legs and thighs are used far more often. This difference in muscular activity means that these muscles have evolved differently. Breasts and wings are dominated by fast-contracting muscle tissue. Thighs and drumsticks are predominately slow-contracting muscles.
In recalling Biology 101, muscles require energy in order to operate. Energy for the fast-contracting muscles comes from glycogen, a carbohydrate in the form of glucose. The slow-contracting muscles fuel comes from fat.
This works differently in birds that can fly. When looking breast meat of ducks or geese, they often has a similar fat content as the legs or thighs.
So in short, dark meat is darker than white because it has more fat. This fat is used to supply energy to muscles that are used on a near continual basis.