The Secret of Soaring Birds: Aloft on Inflatable Sacs
Posted by Richard Conniff on June 12, 2024
by Richard Conniff
I’m watching a red-tailed hawk just now circling above the lower Connecticut River. It’s a placid, sunlit day, not like the mucky, half-drowned day Ted Hughes wrote about in his 1957 poem “The Hawk in the Rain.” But my hawk, like his, “Effortlessly at height hangs his still eye.
His wings hold all creation in a weightless quiet,
Steady as a hallucination in the streaming air …
The hawk’s flouting of gravity fills poor humans like Hughes (and me) with wonder, longing, and also envy, at the contrast with our own earthbound lives. So how is it that hawks and other soaring birds do it? How do frigatebirds manage to remain on the wing at sea for months at a time? How do golden eagles stay airborne for hours while hardly ever flapping their wings? How does the peregrine falcon hang patiently in the sky before its high-speed plunge to the kill?
Researchers have long known that soaring birds ride updrafts in the mountains and thermals wherever they can find them. But a new study in the journal Nature details an improbable anatomical adaption that makes soaring easier in almost any conditions. It’s a pneumatic sac, called the subpectoral diverticulum (or SPD), that extends from the lungs, passes out of the body cavity through a narrow opening at the shoulders, and then dives down between the major flight muscles in the chest. When inflated, this sac becomes a sort of cushion, extending support for the outstretched wings a centimeter or two from the body.
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