Lang Elliott didn’t realize the birdsong he was missing until the Worm-eating Warbler incident. In the 1970s, a professor noted the bird, but even standing beneath it, Elliott couldn’t pick out its lusty, high trills. “I’m watching it throw its head back and open its beak and sing its heart out. And I still don’t hear that bird,” he recalls. This “ear-opener” led to an experiment: Elliott slowed the speed—and lowered the pitch—of a recording he took in a forest. He was shocked at the birds he heard. A test revealed Elliott, then 27, had high-frequency hearing loss, a condition caused by loud sounds or aging that one study found may affect nearly a third of U.S. adults under 70. “I was missing this huge part of the world of birdsong, not to mention insects,” he says, a crushing realization for the budding wildlife ecologist. The severity of his hearing loss above a certain frequency—due to a childhood accident with firecrackers, he realized—meant...