Signs of Spring: Report Your Observations!

Red or White?
People who eat meat notice that there are two kinds: dark and light, or red meat and white meat. What's the difference? And how is the kind of meat in a bird critical for its lifestyle?

A chicken or turkey breast is white meat, while the legs and thighs are red. Ducks and geese have entirely red meat. What's going on?

What we call "meat" is actually muscle. Muscle fibers do work by contracting, pulling wing or leg bones to make them work. The action is similar to the force exerted by a stretched rubber band squeezing your fingers as the band tries to get back to its original size.

This rubber band is not stretched; it's like a relaxed muscle fiber.

This rubber band is exerting a force and pulling on the fingers. But the boy's arm muscles are pulling right back.

To do the work of stretching and then snapping back into shape, muscles require oxygen and energy, provided from the body's blood. Some muscle fibers can work steadily for a long time, repeatedly stretching and pulling, stretching and pulling. When a Ruby-throated Hummingbird flies over the Gulf of Mexico, its wing muscles may do this a million times without resting! This requires a very steady supply of blood. Muscle fibers that need a rich supply of blood and can work steadily without resting are called red muscle fibers.

But all that blood circulating to and from red muscle fibers is heavy, and makes the heart work harder. So when a bird doesn't need to use its muscles very often, but occasionally needs a big burst of activity, it uses white muscle fibers. These work like thicker rubber bands, making a more powerful and fast "snap" when they work, but because there is little blood to these muscle fibers, they must rest once they've done their job a few times. That's because when any muscle fiber works, it releases a chemical called lactic acid. Did you ever get a cramp in your leg after running a long way, or in your arm after pitching for a long time? That cramp was caused by lactic acid that built up in your muscles.

A wild turkey doesn't fly very often, but when it does, it can beat its wings furiously to carry its heavy body away from danger in a hurry. To save energy during the rest of the time, its pectoral muscles are mostly made up of white muscle fibers. After a flight, that turkey is really tired! It won't be able to fly for several minutes while lactic acid is carried away in the blood stream.

Try This!
Think about the differences between red and white muscle fibers. Then decide whether each of these bird muscles is made up of mostly red or white fibers. (Remember: breast muscles power wings and thigh muscles power legs.) Be ready to explain your answers.

Bird Part	Red or White?
1. Barn Swallow breast muscles	.
2. Crane wing muscles	.
3. Crane leg muscles	.
4. Hummingbird wing muscles	.
5. Eagle wing muscles	.
6. Eagle leg muscles	.
7. Red-winged Blackbird wing muscles	.
8. Loon wing muscles	.
9. Loon leg muscles	.
10. Robin wing muscles	. .
11. Robin leg muscles	. .

To check your answers, click here.

Why do you think we didn't ask about hummingbird or swallow leg muscles?

The Human Factor: Questions for Discussion
We humans have muscles made up of a combination of red and white fibers, pretty much the same as birds. Athletes who are extremely speedy — whether running, swimming, or skating — have more white fibers than most people.

Why do you think these athletes are less likely to run marathons or participate in other long-distance events?
Athletes who have great endurance and can "go the distance" hardly ever participate in short-distance events. Why not?
Eric Heiden, a speedskater in the 1980 Winter Olympics, won gold medals in EVERY individual skating event. Use what you just learned about muscles to answer: Why is this such an amazing feat?