At Thanksgiving, many of us are faced with choosing our favorite part of the traditional bird. Some of us prefer the legs and thighs for their dark meat, while others opt for the whiter breast meat. We know which kind of meat we like best, but few of us know the reason for the difference.
It's a question of fibers. The muscles of all birds possess several types of fibers. In fact, each muscle usually has a mixture of these fibers. Because red ones predominate in the leg and thigh muscles of birds like turkeys and chickens, the tissue there appears dark. The breast tissue, meanwhile, is composed mostly of white muscle fibers. Why? Because the breast muscles of these birds serve a different purpose from their leg and thigh muscles.
Turkeys and chickens, remember, are ground-dwelling birds that spend all of their waking hours walking, running, and scratching for food. The muscles in their legs are adapted for constant use and do not fatigue easily. The individual red fibers are very fine and contain an abundance of compounds that make them efficient at aerobic respiration. These tissues are also high in both fat and sugar, which act as fuel for aerobic metabolism. Small songbirds, by contrast, can fly efficiently for hours at a time because they have a predominance of red fibers in their flight muscles.
While turkeys can fly, too, they aren't capable of sustained flight. The large muscles that we call "white meat" are adapted for explosive bursts of power -- such as when one of these heavy-bodied birds is surprised by a predator and must escape in a flash of speed (turkeys have been clocked at 55 miles per hour). These muscles are powered by anaerobic metabolism and tend to fatigue quickly.
The opposite is true of ducks and geese. These birds are long-distance fliers, and their large breast muscles consist of dark meat that's high in fat content. Yet the birds with the greatest concentration of red muscle fibers in their flight muscles are not the ones that make the longest migratory flights. No, it's the birds that utilize the highest number of wing beats during sustained flight. And which birds are those? Hummingbirds, of course.