For the Birds Radio Program: Respiration
Several years ago, I watched an IMAX film about climbing to the top of Mount Everest. Near the top, most of the people were using supplemental oxygen, and according to the film, few ever reach the top without an oxygen tank. At one point, as the narrator talked about how cold, exhausted and oxygen-depleted climbers are when they approach the final two stations, in the background, apparently unnoticed by the filmmakers, were a small group of Alpine Choughs, relatives of our crows, raiding the camp’s food stores. These birds had flown there entirely on their own power, naked as jaybirds and clearly without supplemental oxygen.
Many migrant birds that cross extended areas of the ocean fly at 12,000 feet at nighttime when there are favorable tailwinds at high altitudes. These are birds that spend their daily lives at considerably lower altitudes, and they don’t go through any conditioning exercises before migration to build up their lungs. Flying at 12,000 feet is a paltry accomplishment compared to the Bar-headed Geese that cross the Himalayas at 29,500 feet, or the world-record holding Ruppell’s Griffon, a vulture once recorded at 37,000 feet. A Mallard which struck an airplane at 21,000 feet holds the record for the highest documented flight altitude for a bird in North America, but the poor duck didn’t live to celebrate the accomplishment.
Except for the record-holding vulture, these high-flying birds have to exert themselves with steady wingbeats even at extremely high altitudes. Humans and, indeed, all other mammals, are incapable of this kind of exertion where oxygen is so minimal. How do birds manage it? They have the most highly-developed respiratory system in the animal kingdom. We mammals have huge lungs filled with dead-end sacs called alveoli, where gas exchange with capillaries takes place. When a bird inhales, air sacs behind the smaller, flatter lungs draw fresh air from the trachea across the surface of the lungs, which rapidly pull out oxygen from the tide of air, while during that same inhalation, air sacs anterior to the lungs pull the carbon dioxide and stale air away from the lung’s surface. When the bird exhales, the posterior air sacs push their fresh air back into the lungs for gas exchange while the anterior air sacs exhale the stale air to the trachea and out. There are no alveoli, or “dead spaces,” in bird lungs. The unidirectional flow of air across the gas exchange surface is what makes bird lungs so exquisitely efficient.
Bird lung design has a second huge advantage for a creature that needs to fly. Our enormous, air-filled, low-density lungs are situated in our chest, right where birds need the densest mass of tissues for proper balance in flight. By having flat lungs designed for gas exchange without needing to also hold the air as our alveoli do, birds can keep the area between their wings filled with their densest organs. Bellows-like air sacs fill other spaces, particularly in their lower abdomen and in hollow spaces in their neck and inside the humerus bones. When a bird dies, those relatively huge air sacs empty, and the bird’s body literally shrinks. But while it’s living, a bird’s every breath bears testament to a respiratory system as close to perfection in form and function as anything the animal kingdom has to offer.