Every night beginning in late March and into early September, nearly 1.5 million Mexican free-tailed bats emerge from the Congress Avenue bridge in Austin. The bats aren’t the only thing making a nightly exit, though: Bat guano, otherwise known as poop, makes its exit as well.

Catherine Samuel was curious about where the guano goes and who cleans it up, so she asked about it for our ATXplained project:

Does anyone ever clean out the bat guano from under the Congress Avenue bridge?

She says her curiosity sprouted from a childhood memory.

“When I was little, my dad really wanted to attract bats, because he really wanted to get rid of the mosquitos around the house," she says. "So, he installed a bat house over the garage and he bought a bunch of bat guano and smeared it on the ground to attract the bats."

That memory would follow her into adulthood. Samuel said she became keenly aware of the smell of bat guano while interning at the Texas Capitol, and this eventually led her to become interested in the bridge.

To answer Samuel’s question, I had to first find out who maintained cleanup of the bridge. I spoke with the Austin Parks and Recreation Department, which directed me to the Texas Department of Transportation. When I spoke with the Transportation Department, the person on the phone pointed me back to Parks and Rec.

If neither the city nor the state is in charge of cleaning up the guano, then who is?

For more than 10 years, the Austin Bat Refuge has been rehabilitating bats, as well as educating people who visit the bridge. Lee Mackenzie, its co-director, said the group has taken a particular interest in the feces because it can make for “amazing” fertilizer.

The nonprofit has placed plastic sheets under the bridge to collect the guano. In about four hours, it collected 26 grams – just enough to fill up a small baggy.

“When you pick it up, it’s beautiful. It’s the color of all the insect carcass that they have been eating all night," Mackenzie says. "It can be iridescent and glow, and it has reds and greens and blues [and purples]."

The bats consume 200 tons of insects a night. Mackenzie says these insects consist of the corn earworm moth, one of the worst crop pests in North America.

Mackenzie says his group doesn’t collect guano from under the bridge often, because it’s not as valuable as guano from caves. That guano contains a higher nitrogen content, creating a powder better suited as fertilizer. The guano that drops just a few feet from the bridge is still valuable for some of the plants, he said.

“It is perfect fertilizer for the ragweed that grows here," Mackenzie says. "We have the most abundant and wonderful ragweed crop on the planet – if you like ragweed."

It’s understandable that tourists could be concerned about coming in contact with a bat and its feces. Bat droppings can cause histoplasmosis, an infection caused by breathing in spores. The Centers for Disease Control and Prevention says, however, that not a single case has been reported in Texas.

In fact, Mackenzie jokes, bat urine – which his group calls honeydew – can bring good luck.

"The thing about that is if you simply buy a lotto ticket before you wash it off, you are guaranteed to win,” he says.

So, there really isn’t anyone designated to clean up the guano underneath the Congress Avenue bridge. If the guano isn’t falling into the plants under the bridge, it simply gets washed away by the rain. It also falls into Lady Bird Lake, which doesn't impact water quality, according to Bat Conservation International.

If you want to catch these bats in flight, you can park at the Austin-American Statesman Bat Conservation Center until October, when the bats migrate to their winter homes in Mexico. In summer, the bats typically leave the bridge right after sundown, so it’s best to get there before the sun sets.

And maybe if you’re lucky enough, you’ll get a little bit of honeydew on you.

Correction: An earlier version of this story said nearly 1.5 billion bats live under the bridge. The correct number is 1.5 million.

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