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The 1.5 million Mexican free-tailed bats that live under Austin’s Congress Avenue Bridge in the summer are a welcome success story of urban wildlife conservation. Once dismissed as pests, the city has come to embrace the bats as allies in the fight against mosquitoes and as little-winged moneymakers when it comes to attracting tourists.
These days, the creatures generate up to $10 million in bat-related visitor dollars annually, according to Texas Parks and Wildlife. An estimated 140,000 people come to see the bats pour out from under the bridge in their awe-inspiring evening flights.
But there is a catch. The bats don’t always show up.
Some nights, they leave their roost after it gets dark, and you can’t see them well. Other nights, they don’t leave the bridge at all.
Whether and when the bats decide to fly is determined by many things, including the time of year, the weather, and longer-term climate patterns.
What conditions will improve your odds of catching the natural wonder that is the Austin bat flight?
That’s the question KUT put to some of Texas’ most respected bat experts. Here's what they said:
Summer is prime bat season
The bats that occupy the Congress Avenue Bridge, also called the Ann Richards Bridge, are migratory animals. They typically start arriving in Central Texas around March from their winter homes in Central and South America. They have babies and raise their young in Texas, then return south by November.
“A lot of the big conglomerates [of bats] that we see are actually maternity colonies,” says Sarah Fritts, a professor and bat researcher at Texas State University. “They all come together to give birth, typically to one pup each, and raise their pups together.”
The bat pups are typically born in June, but it takes four to five weeks before they take flight. That means you are likely to see the most bats later in the summer when mothers and babies fly out together to hunt for bugs at night.
By August, under the right conditions, around 1.5 million or more bats can be seen emerging from the bridge in massive flapping waves.
Rainy weather is bad for bat viewing (but good for the bats!)
Similar to most humans, bats don’t like to go out in the rain. One reason is that their food is not as easily available.
“These are insectivores. So, they're chasing bugs at night,” says Fran Hutchins, with Bat Conservation International. “If it's raining, basically, the food isn't flying around, so the bats won’t come out.”
Even if bats don’t like to fly while it’s raining, wet weather is beneficial to them. That’s because rainy weather boosts insect populations. So, there are more bugs to munch on when the bats exit their roosts on drier evenings.
There is a catch, though. Bats with full bellies tend to wait till later at night to go bug hunting. If they’re really stuffed, sometimes they don’t go out at all.
That means that during particularly rainy springs or summers, the bats are more likely to stay in for the night or to come out after sunset, when viewing them is more difficult.
“People call me and ask ‘What's wrong? Have the bats died? We don't see them this year!’” says Merlin Tuttle, a world-renowned bat expert and conservationist. “No! The bats didn’t die. They're having a wonderful banquet, and they just don't need to come out early this year!”
There is a flip side to this phenomenon.
Drought, or even just drier weather, means there is less food for the bats. Hungry bats are likely to leave their roosts when the sun is still out to have more time to hunt for bugs.
That’s good for bat viewing, because you can see them when it’s light out, but it’s not necessarily good for the bats. It means they’re hungrier and they’re more vulnerable to predators like hawks and owls.
Storms, and heavy winds, even far from the bridge, may keep them in
The bats that emerge from the Congress Avenue Bridge can fly hundreds of miles seeking food in a night, but they don’t want to do it in bad weather.
Experts say if the bats sense a storm brewing, even far from their roosting place, they may stay in for the night.
“Even if we don't see it at the bat viewing area, they know there's a storm out there,” says Lee Mackenzie of Austin Bat Refuge. “So they're not going to take a chance coming out if they could get pounded by hail and break their wings.”
If you happen to find an injured bat. You can call the Austin Bat Refuge for advice on what to do next. Start on this website, where the organization provides plenty of advice and contact information.
The Bat Refuge also shares daily forecasts on its website advising on how ripe conditions are for a good bat emergence.
“We collect 100 data points a night," Mackenzie said. “We don't have a fancy algorithm. We watch the trends during the week and we weigh the trends against the daily weather.”
Hot weather is better for bat viewing, especially at the bridge
Bats don’t like cold weather, or even cool weather.
“Bats can actually go into torpor when it's anywhere from the 50s to sometimes, some species, 70 degrees,” Fritts says.
A torpor is “a tiny, short bout of hibernation that slows their metabolism down,” she says.
It means the bats are not leaving the bridge.
Tuttle says extended cooler temperatures may also prompt some Congress Avenue Bridge bats to seek warmer roosts like the famous Bracken Bat Cave Preserve outside of San Antonio.
But, when temperatures spike, bats can get too hot as well.
Tuttle believes that when summer temperatures get dangerously high, more bats leave the Bracken cave and relocate to bridge roosts like the Congress Avenue bridge.
“They come out here [to Austin] when it stays really consistently hot,” he says.
Bats still put on a great show after sunset, if you watch from the right place
If you happen to visit the Congress Avenue Bridge on a night when the bats take flight after dark, don’t despair.
The view can still be impressive, if you watch from below.
Typically people like to stand on top of the bridge to watch the bats. That makes sense on days when they emerge while it’s still light out. The view from the bridge allows you to see the millions of bats snaking downriver in giant ribbons of life.
But, on days when bats are emerging close to dark, the bridge is the wrong place to be, says Mackenzie.
“You won't see many bats, you'll just see dark bats against dark water,” he explains.
Once the sun sets, he says, it's better to go down below the bridge along the shores of Lady Bird Lake.
The hill on the southeast side of the bridge is one popular place to observe.
Others take kayaks, canoes and paddleboards to get right under the bridge. Though the view, again, depends on where you position yourself.
Buying a ticket on a bat tour boat is another way to get out on the water.
“You can backlight [the bats] against light in the sky or the streetlights, or the buildings and still get a good view,” he says.
The Congress Avenue Bridge is also not the only place to watch bats take flight in the Austin area.
From the Bracken Bat Cave Preserve, which requires you make an advance reservation, to the McNeil Bat Bridge on I-35 in Round Rock, Texas, there are plenty of opportunities to watch.
No matter where you choose to see them, you're sure to have a story to tell. Even, if it's about that time you got stood up by 1.5 million bats on a rainy summer night.
KUT's Nathan Bernier contributed to this reporting.
