I was watching a skein of geese flying south for the winter, individuals arranged in a typical V-shape, each bird pumping its wings up, down, up, down, hour after hour after hour. Just watching them, knowing how many hundreds, thousands, of miles they had to go before they reached their destination, made me exhausted.
Then there are the songbirds, who migrate so high that we don’t even notice them. How do they travel such long distances, exerting themselves where the air is so thin? Even more, they can fly and vocalize at the same time. (When I’m hiking at high elevations, it’s all I can do to gasp for air; I can’t even talk, much less sing!)
Wondering turned to some research, and I learned at least one bird secret for sustained, high altitude flight. Their lungs don’t work like ours do.
Like all mammals, we breathe in, then breathe out, then in, then out, in, out, in, out. We can develop a larger lung capacity, or up the rhythm when we need more oxygen, such as during exercise, but there is a limit to how fast we can breathe. All that air that we breathe in has to be pushed back out again before there is room for a new lung-full. Given that we don’t completely empty our lungs with each breath, fresh air mixes with the remaining stale air, reducing the efficiency of each inhalation.
Birds don’t work that way. Like us, they have nostrils, a trachea, and two lungs, but they also have air sacs (nine in most species). Birds also have hollow, air-filled bones. These air spaces are all interconnected. While the air sacs and bones aren’t involved in gas exchange as the lungs are, they do serve a purpose.
When a bird inhales, air goes to the “incoming” air sacs (in the body) and the lungs, while the air already in the lungs moves to the “outgoing” air sacs (in the head). When the bird exhales, the air in the incoming sacs moves to the lungs, while the air in the outgoing sacs is expelled through the nostrils. Inhale again, new air enters the lungs and incoming sacs, while stale air moves to the outgoing sac. Exhale, and the lungs get the fresh air waiting in the body’s air sacs, while the stale air all goes out through the nostrils. Thus it takes two breathes to move air all the way through the system.
The benefit of this arrangement is that air moves in a circle, rather than back and forth. Both inhaling and exhaling bring a full load of fresh air to the lungs. In fact, it’s almost a continuous flow of air, allowing much more oxygen to be absorbed and carbon dioxide to be eliminated.
To add to a bird’s respiratory effectiveness, its air capillaries are ten to a hundred times smaller than our alveoli, with thinner membranes, allowing for more efficient gas exchange.
This “flow-through ventilation” also helps explain how birds can call while flying, or sing so long without seeming to stop for a breath. Because they can control their lungs and air sacs independently, they can breath in and out at the same time. When expert musicians practice “circular breathing,” they’re merely mimicking what the birds have been doing for ages.
This respiratory system is so efficient that birds breathe much more slowly than mammals of comparable size. No wonder they can perform such amazing physical feats!