Bird Muscles and Migration

Emily Stone

Sometimes it takes a lot of willpower to put on my running shoes and take off down the road after a day at my desk. Today, despite (or maybe because of) the intermittent cloudbursts and beckoned by the ovenbirds’ rapid-fire teacher-teacher-teacher songs ringing through the trees, it felt like a necessity. As the cool drops misted my face and the raindrops swallowed sunlight while abandoning themselves to gravity, I felt multiply rewarded for my effort.
“What makes aerobic exercise so powerful is that it’s our evolutionary method of generating that spark,” according to Dr. John Ratey. “It lights a fire on every level of your brain…”
Then, to add to the magic of the moment, the flute-like song of a wood thrush floated sweetly above the treetops. Henry David Thoreau wrote, “The thrush alone declares the immortal wealth and vigor that is in the forest. Whenever a man hears it, he is young, and Nature is in her spring.”
A few more minutes down the road, and a bird flew about five feet in front of my nose. I stopped to look as it landed at eye level. Bird watching (and listening) is one of my favorite parts of running. Even without binoculars I could tell it was a yellow-rumped warbler, yet another early spring migrant. Soon a whole flock moved through the branches.
The ovenbirds, the thrush, and the other warblers are in the midst of incredible, ancient migrations from their wintering grounds in Central America to their summer nesting territories in the U.S. and Canada. Their ultramarathon skills put my little four-mile jog to shame, but then again, they are better designed for it.
Birds save energy during flight by having short humerus (upper arm) bones and lengthening their forearms with featherlight structures that are, well, feathers. You can appreciate the difference this makes when you switch from heavy winter boots to your light summer sandals. Every extra ounce your leg has to lift makes each step that much harder! So, light, hollow wing bones, as well as having the wing muscles located on the chest, concentrates weight close their body and their center of gravity.
By selecting easily digested foods like berries, seeds, and insects, birds were able to shrink their stomachs, too. (Just compare a bird gut to the gut of an herbivorous cow.) Birds also traded heavy teeth and jaws for lightweight beaks.
With less room needed for their digestive tract, birds have room for more air. Their unique lung system allows birds to route air through their lungs, instead of in and out like we do. This makes their fresh air exchange incredibly efficient, and allows species like bar-headed geese to fly over the top of Mount Everest—without supplemental oxygen!
In the fading sunlight, the flock of warblers flitted effortlessly as I huffed and puffed down the road. I confess, I have a bit of lung envy.
But it isn’t just that they take in fresh air more efficiently. The hemoglobin in birds’ blood has a high capacity to bind oxygen. Their blood actually delivers more oxygen per unit than ours. Another chemical, myoglobin, takes up oxygen from the blood and delivers it to the cells. This dark red protein is what colors the dark meat on your Thanksgiving turkey. The combination of all these things gives birds an incredibly high VO2 max, a measure of aerobic capacity that will sound familiar to many skiers.
Not only do I wish I had their lung capacity, I also wish I could eat like a bird. Right before migration, birds feast like crazy, and can double their weight in as little as ten days. And it’s healthy for them! The fat gained during this period of “hyperphagia” is stored in masses under the skin on the abdomen and upper chest. It can fuel nonstop flight for three days and nights. Then the birds are lean again.
While birds have adaptations for athleticism that I can only dream about, we do share one behavioral adaptation: both the birds and I choose to exert ourselves when it is cooler to avoid dehydration in the heat of the day.
“Birds are not likely driven by the logic of what they do,” biologist Bernd Heinrich philosophizes in his book, Why We Run, “instead, they are motivated by powerful urges. They behave in ways that feel right and pleasurable to them. Feelings of pleasure are a byproduct of evolution that makes healthy organisms do what helps them survive and produce offspring, in the same way that fear makes them shy away from danger.”
What makes birds begin their migration, Heinrich says, “is probably not fundamentally different from what motivates me to jog down a country road on a warm sunny day.” And, by the end of our respective migrations, both the birds and I feel like singing.

For over 45 years, the Cable Natural History Museum has served to connect you to the Northwoods. Come visit us in Cable, WI! The current exhibit, “Nature’s Superheroes—Adventures with Adaptations,” open until March 2015.

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