Eagles and the Genius of Bird Flight

I’m not sure how I see them. It is the most turbulent day of winter yet. Wind tears through the firs that surround Key Center. I scuttle across the highway and aim for Capitol Lumber, eyes down, hunched against spitting rain.

Nonetheless the eagles catch my attention. There are three working the sky to the east. Among scraps of low clouds, they buck and wheel, twist, rip downwind with awesome speed. I realize they are playing catch with the gusts, wrapping their huge wings around them and hurtling up and down like trapeze artists.

I watch for a few minutes. At one point 100 robins leap from trees to the north and, battling like fish in a current, attempt to cross town. A group of 20 are flipped around and sent flying back the way they came.

When I come out of the lumber store the eagles have been replaced by gulls. On slender wings they arc and flash. Again, I watch. The rain is just as harsh as before but somehow it now feels invigorating. I think of my most miserable days on the coast when the wind is so strong that waves fracture at their crests and saltwater pummels the forest. Even then the gulls are aloft above the waves, electrified, dipping into troughs and getting soaked, letting themselves be carried up and away. Like albatrosses and shearwaters, fellow seabirds that cross hundreds of miles of ocean without flapping once, gulls know that wind charges the sky with possibility.

Eagles might feel the same, I’m thinking. Normally their flight is labored. Today they cavort.

And these are conditions that would keep a small aircraft grounded. For how much people have looked to birds for inspiration when it comes to designing flying machines, the exact mechanics of bird flight have remained surprisingly elusive, particularly on turbulent days like this. A thousand feet above me, the wind is likely a smooth stream of rushing air. Here, near the surface, where hills and trees fracture the wind into eddies and slipstreams, shearing it at odd angles that conflict and fight, micro-gusts cannot be predicted. They can only be reacted to, and that is the genius of bird flight.

The most obvious parts of a bird’s flight system are its feathers and musculoskeletal system. Feathers create a contoured shape to catch and cut through the air. Pectoral muscles attached to a keel of bone along with hollow bones forming wings create the ability to flap and glide. Scientific models have focused on one or the other: aerodynamics or biomechanics.

But it’s where the two meet that things get interesting and so complex they prove impossible to track in a living bird, much less recreate in mechanical form. 

Three types of muscles connect wing bones to feathers, uniting them in guilds called feather tracts, interconnecting the tracts, and in some cases giving special control over individual feathers. These muscles can be very difficult to find in dissection, as they are often hidden in elastic ligaments. One study calls the whole setup a “black box.” As a bird reacts to small perturbations in the wind, it shifts its feathers and their tracts to alter its wing shape in endless permutations. A bald eagle has over 7,000 feathers.

No wonder we cannot model the “aeroelastic” flight of birds. While our science and aircraft are built upon the control of variables, wild creatures respond to variables, using and dodging them as naturally as we catch our balance when we stumble. Driving home, two ravens blast through the treetops. They’re the ultimate storm-bolt of a bird, coming alive in the most adverse conditions, laughing, hitting branches and doing barrel rolls as they go.

But today it’s all about the eagles. At a beach near home, I watch as another three appear over a far shoreline. Without flapping or losing altitude they come soaring overhead with feathers constantly adjusting. With a start I realize the wind is at my back. They are soaring upwind, 45 degrees off the headwind, with the magic of sailboats.

Later, over a large beaver pond there are eight eagles in constant motion. Others pass by on the horizon. Washington sees an influx of thousands of eagles each winter. How many are on the Key Peninsula today?

The eagles over the pond swoop up at each other in loose pairs, flapping hard to draw close high in the air, almost courting. I wonder if I am witnessing the reunion of our local mated pairs. Recent GPS tracking of eagles has revealed that many of the eagles that breed in western Washington have an interesting reverse migration. In late summer, after they have left their newly flying offspring to their fates, they head north and spend several months as vagabonds, poaching fish in rivers and lakes all the way into Alaska and the Yukon. Besides there being more fish farther north, many of the runs are earlier than our own.

And mated pairs, which mate for life and use the same nest year after year, do not necessarily travel together. One of the first pairs to be outfitted with satellite trackers were from the Skagit River. The male followed the coast north until he found rich herring pickings off Prince Rupert, near the Alaska panhandle. The female, however, crossed three mountain ranges to the east and made it all the way to Great Slave Lake in northern Alberta, where she probably hunted Northern pikeminnow.

Eagles such as these return as fall turns to winter, when turbulence fills the sky and freshets flush the creeks. That is when our salmon come, and that is when mated eagles renew their pair bonds with displays like those today, which last well after sunset.

Again and again the eagles turn into the windstorm and let its violence carry them higher, higher. They flap hard to nip at the tails of their mates. They flip on their backs and flash their talons. Though I do not witness it today, sometimes these teases, which blur the line between play and power, turn into the ultimate bald eagle display, when two eagles grasp talons midair and spin around each other in freefall, cartwheeling out of the sky, letting go to glide away mere feet from the ground.