I was minding my own business at the desk by the window when WHACK, a Cardinal crashed into the glass. I rushed outside to look for a body in the hedge, or at least a stunned bird, but found nothing, not even a red feather. He must have survived. It got me thinking about flight. It’s marvelous and amazing and we terrestrial-bound species are jealous of the birds, but it does come with risks and at a price. What are the risks and what exactly have the birds given up when they evolved this specialized skill.
I count five groups that have acquired the ability to fly, (omitting the gliding frogs and squirrels). They are the myriad insects, the extinct dinosaurs–Pterosaurs, the mammalian bats, the birds, and Homo sapiens, since Kitty Hawk. You must admit that at least with insects and birds, flight has been a successful strategy, with Aves flying around for 150 million years since Archaeopteryx, and insects for even longer. This compares with a meagre 20 million years for Hominids on earth, with flight mastered by us just 115 years ago.
There are, of course, obvious advantages of bird flight. They can get from point A to point B quickly, whether its to find food, escape a predator, or chase a prospective mate. The destination may just be across the yard or a migration of thousands of miles. Their flying skills include, hovering, take-offs and landings, on either land or water, soaring, gliding, and high speed dives. They can catch a fly on the wing and even copulate in mid-air. Very impressive.
There are, however, obvious physical risks to flight. My office window, multiplied by millions is an example. Add to that the glass of towering skyscrapers, burgeoning wind farms, and power lines, and you have some real flight hazards. Fall migration itself takes a huge toll on the young birds. That’s why the spring migration is less crowded, returning to us just the survivors.
But I’m more interested in the anatomic and physiologic adaptations that have evolved and made flight possible, and what price Aves have paid for this specialization. The upper extremity of birds has reduced the five digits of its ancestors to three and these serve as the anchors for the primary flight feathers. The wing is a wonderful and highly specific adaptation for flight, but useless for grasping a tool or playing the piano. No matter; birds have evolved a flexible neck and versatile beak and tongue to partially offset these deficits.
What about size? It does matter for birds. Flight requires the birds to be relatively small and light. When you double the length of a bird you increase its weight 8-fold. Even though the large Golden Eagle only weighs 15 pounds it requires an 8-foot wingspan to fly.
The physics of flight applies to the birds, just as it did for the Wright brothers. There must be air flowing over the wing or airfoil to create enough lift to overcome the drag. Flapping adds greatly to the lift, but weight is still a limiting factor. Just recall the spectacle of the heavy swan or goose, beating its wings while running across the pond, in its onerous fight to become airborne.
Experts debate how the Pterosaurs and ancient birds “learned” to fly. One camp suggests a “tree-down” approach, falling or gliding from a height, similar to flying squirrels. Another group suggests a “ground-up” technique, running or leaping into the air, similar to our struggling swan. I doubt we’ll ever know for sure.
Birds have also solved the weight issue by their light, hollow bones, ideal for flight but lacking somewhat in strength–another compromise. “Light as a feather”, the saying goes. The evolution of the feather figures centrally in the history of flying animals. Experts now believe feathers evolved long before flight. Once we pictured dinosaurs as hairless, leathery reptiles, but now learn that some were actually adorned with colorful feathers. The only question is whether their feathers were for insulation or for sexual ornamentation, but clearly they were not, at least initially, useful for flight.
The weight restrictions of flight also require that a bird brain remain relatively small, and surrounded by only a thin skull. Most of its brain is devoted to eyesight, so highly perfected in raptors, and much of the rest to the regulation of basic functions and the intricate movements of flight. Although much has been written about the intelligence of birds, (primarily the Corvids), don’t get carried away. They will not be writing a Beethoven symphony any time soon, or even running for political office.
The warm-blooded, hyperactive, flying birds are massive consumers of energy. Their high metabolic rates require a never-ending search for food (using energy in the process) for both themselves and their young. It is a bird’s greatest mission everyday. The avian respiratory system is also a unique and complicated adaptation of rigid lungs, multiple air sacs, and unidirectional air flow, all designed to supply richly oxygenated blood to meet their high energy demands.
It’s interesting that some birds have given up flight completely. You wonder why. For Penguins the rudimentary wings are now used for swimming, while the large Ostriches of the savannas of Africa use their downy feathers and wings for shade. The flightless Dodo birds of the Mauritius Island in the Indian Ocean were doing just fine on the ground until discovered by Dutch sailors in 1598. The vulnerable bird was easy prey for man and his contaminants and the Dodo is now extinct. Unfortunately its name has become synonymous with naiveté and stupidity.
So the birds have paid some price for their lives in the sky. We humans need to keep this in mind as we stretch our frontiers upward, even to the Moon and Mars. I consider Homo sapiens now a flying animal, similar to the birds. We are part of nature and not just an outside observer looking in. Never mind that our “wings” are metal and rivets and computers; they are merely our adaptations, the products of our brains, and our unique ticket to the wonders of flight.