The Traveling Naturalist, our series featuring natural wonders and biological curiosities for the science-inclined wanderer.

What’s the world’s largest concentration of mammals? Many people guess that it’s one of the great herds—the wildebeest in the Serengeti, or caribou in the Arctic.

But no: To see even more mammals, you have to look to the sky. More specifically, head to a bat cave in the Texas Hill Country, between now and the end of summer.

At caves around Texas, Mexican free-tailed bats emerge nightly by the millions. Yes, millions. The biggest? Bracken Cave, owned by the excellent conservation organization Bat Conservation International, with an estimated 15 million bats. That’s a lot of critters.

These are maternal roosts: females come here to have young. Come fall, they migrate south to Mexico.

Bats emerge en masse from caves, and within minutes they stretch out to the horizon. At a glance it resembles nothing so much as a thick cloud of smoke, swaying in the breeze.

Where there are large congregations of animals, of course, there’s also congregations of predators to take advantage of the bounty. Hovering outside the caves are often a variety of raptors, ready to snatch a wayward bat.