The bat cave may be closer than you think.
Every night at this time of year, huge clouds of Mexican free-tailed bats swirl out from under Tucson bridges at sunset and fly into the twilight sky.
“Almost every major bridge in Tucson has a colony of bats underneath,” bat expert and former University of Arizona mammal museum curator Yar Petryszyn said.
He estimates there are 20,000 bats living under the Pantano Wash bridge on East Broadway, and up to 200,000 total bats flying around Tucson this summer.
The free-tailed bats migrate north to Tucson in April to feast on an abundance of insects and will return to Mexico in October, Petryszyn said.
Each bat weighs on average about 13 grams, with a 10-inch wingspan and a body the size of a human thumb.
A single bat will eat half its body weight in insects every night, and it is estimated that the Broadway and Pantano colony will eat 2 tons of bugs a season, Petryszyn said.
Thousands of bats take flight as soon as the sun sinks below the horizon, creating a nightly spectacle with a popular following in Tucson.
Just before sunset, Valerie Duran, 32, and her two children venture as close as they dare to the bridge to see the 20-minute “bat show.”
“Everyone brings their kids to see it,” Duran said. “My daughter was getting scared, asking, ‘Are they going to suck my blood?’ ” she said.
Mycah Duran, 6, and Diego Duran, 13, didn’t seem afraid as they sat on the concrete slopes of the Pantano Wash pointing overhead and saying things like “Whoa!” and “There must be a thousand of them!”
The expansion joints of Tucson’s bridges provide a perfect home for the bats. The 1-foot deep grooves run along the length of the bridges and give the bats an ideal location to raise their young because the concrete crevasses remain at a consistent temperature, Petryszyn said.
“It’s something you can’t see anywhere else,” bat watcher Adam Freden, 35, said. It was his second time watching the show, and he was prepared with a camera to snap pictures and brought a friend to witness the photo-taking.
Ben Strout, 21, was impressed that there were actually as many bats as Freden predicted. “There should be more things like this where wildlife can be seen,” Strout said.
When the bats are roosting during the day, their high-pitched chattering can be heard near the bridges, but their nighttime echolocation used for hunting is out of the human hearing range, Petryszyn said.
The swarm of bats may seem intimidating, but Petryszyn said they are not dangerous and would only bite to defend themselves if attacked. During his career, Petryszyn has handled thousands of them.
Some bats in the northeastern United States have been dying of a mysterious disease known as “white-nose syndrome,” but Petryszyn said the Tucson bats have shown no signs of the disease. The only problem he has seen is the occasional case of rabies, which he said people have “blown way out of proportion.”
Petryszyn simply cautions people to not touch bats that are lying on the ground.
Bats should not be touched or disturbed because they are a special class of wildlife protected by state and national laws and it is illegal to harm them.
Petryszyn, who has given many lectures about bats at Sabino Canyon, at the Arizona-Sonora Desert Museum, and to civic groups, said bats are to be admired, not feared.
“People are pretty receptive (to bats) with education,” he said.
“Everybody is interested once they learn how valuable they are and what they really do.”
Best places to see bats
• East Broadway bridge over the Pantano Wash
• North Campbell Avenue bridge over the Rillito
• East Tanque Verde bridge over the Rillito
The bats start flying at sunset (about 7:30 p.m.) and usually fly north.