As the sun sinks into the sky on a warm August evening, a black swift nimbly plucks flying ants out of the air. The balloon-like bolus at her neck bulges with the insects, which she’ll soon feed to her hungry chick hidden away in a mountain cave. Her scythe-like wings and torpedo-shaped body blink in and out of view as she rockets toward a 1,500-foot-deep canyon here in western Colorado’s Flat Tops region, where 10,000-foot-high plateaus are riddled with water-carved caves. She’s heading for one of them: Fulton Resurgence Cave, a teardrop-shaped cavity high on a cliff. Water pours from its mouth, bathing the vibrant green moss and algae growing on the walls in mist as the stream cascades into a steep gulch. It’s the sort of place only spelunkers—or possibly hard-core birders in search of the continent’s most mysterious avian species—would venture. The swift makes a tight circle before zipping inside, where her downy chick waits in the dark on a teacup-sized moss...