So, we hunt. Armed with nets, long bamboo poles, and shorter metal poles, our little group traipses through the close-shaven grasses of the Great Meadow, down the rocky path that abuts Olmstead's manmade waterfall, and toward the bat-friendly underpass. One of the Central Park Conservancy members has seen a roost there, Medellín says, and he's excited. A roost would be its own kind of treasure. "I'm thinking they're brown bats -- it's a common one," he tells me as we walk, referring to the big browns. "We'll try to get a count of how many: how many bats are living in that roost," he says. "And if so, that would be good information for the park. And to protect the bats, you know."

Looking for the roost (Megan Garber)

We arrive at the site of the roost. We're nearing dusk. Medellín extracts a small flashlight from one of his equipment bags. He climbs onto a rock. He shines the light onto the concave wall on the other side of the stream, moving the light toward the corners and crevices. He moves quickly at first, then more slowly. He crouches. He adjusts his angles. He streams his light onto every inch of the wall. He does it all over again, just to be sure. He turns to us.

"There's nothing here," he says. He sighs.

Setting the Trap

And that's how Rodrigo Medellín, world-renowned bat ecologist, ends up shin-deep in a stream in the middle of Manhattan, dainty dimes of pond scum swirling around his legs.

Wading in the water, net in hand (Megan Garber)

Since the bats haven't come to us, we're going to go to them: We're going to set up nets to ensnare them. Medellín is carrying a stake and one half of a large net -- one that, at about three feet high and many more feet wide, is black and filmy and slightly sticky to the touch. It's called a mist net, technically, and it's been designed for the specific purpose of capturing bats as they fly. Bats navigate using echolocation; a strategically placed net, a BioBlitz ecologist explains to me, can ensnare the bats' feet as they change direction in their attempt to avoid a collision. It's a tricky little system that uses bats' keen navigational intelligence against them -- but it's one, Medellín points out, that is required if humans want to capture (and thereby study, and thereby help) the animals. The mist net, he says, "is a standard method all around the world."

So Medellín, still in the stream, delicately strings the rings of the net's outer edges onto the pole. He plunges the pole into the silt, balancing it against the shiny rocks that jut out of the water. He shakes the pole, frowning. "I don't like this," he says, as the net sways gently in response. "I want to make it tighter." He adjusts the net.

Across the stream, on the paved part of the underpass, Medellín's assistant, Angelica Menchaca, is making similar adjustments. Menchaca is young -- in her 20s -- but she and Medellín have by now worked together for years. Before she was a grad student at Columbia, she was in his class at the University of Mexico. "This is my student," Medellín says by way of introduction, "slash assistant, slash --"