7-24-08 The Bats of Congress Avenue Bridge

Well, since the subject was bats…

The residents of Austin, Texas got a little surprise when they reconstructed their Congress Avenue bridge in 1980. The underside of the reconstructed bridge had lots of narrow, deep openings, which turns out to be perfect roosting conditions for the Mexican free-tailed bat. So they moved in…

All 1.5 million of them.

You’d probably never know it, though, until sunset, when they fly out to eat. This becomes truly impressive in the summer, when they migrate into under the bridge from their winter roosts in Mexico.

This is what 1.5 million bats look like, courtesy of TRSLLC. Those black puffs in the sky aren’t smoke. They’re thousands of bats.

You’ll probably notice that there’s a lot of humans watching the bats emerge. That’s because the Congress Avenue Bridge colony, which crosses Lady Bird Lake, is the largest urban colony of bats in the world, and the emergence of so many bats at once is the kind of spectacle that tourists like to take in. There are boat rides down the lake at sunset and there’s even a music festival called BatFest.

In fact, why am I telling you this? The Austin tourist industry does a much better job than I ever could. mostlymark shot this from one of the bat-watching boats on April 18, 2008.

It’s amazing, really, that Austin still has this colony 28 years later, and indeed the residents of Austin wanted to get rid of the bats when they heard that they’d moved in. I don’t how they did it, because people tend to react to bats the way Gandalf reacted to the Balrog, but Bat Conservation International (BCI) and other bat lovers persuaded the city not to eradicate them.

The benefits from the Congress Avenue Bridge bat colony are numerous, even beyond the mad tourist jack that they help Austin to rake in.

The Mexican free-tailed bat is a little fella – with an average length of three inches and weighing just over half a pound. But they form some of the largest bat communities – indeed, some of the largest mammal communities – in the world. Mexican free-tailed bats, like almost all bats[1], are innocuous critters, eating only insects, but colonies that size need lots of food – around 10,000 to 20,000 pounds of insects a night.

Think about that. That’s at least five tons worth of free pest control. Five tons of bugs that won’t eat crops. Five tons of insects that won’t bite humans and transmit disease.

Additional reading at:

• Mexican free-tailed bats information at The Wild Ones and the Organization for Bat Conservation
• Pages on Congress Avenue Bridge at BCI and Wikipedia
• Local info on the bats from Austin City Guide and Austin360.com

[1] Vampire bats only rarely feed on humans, and they are blamed for loss of livestock in Latin America, mostly due to infection after being bitten and rabies. I’ve got to believe that the loss doesn’t compare to the scale of true carnivore predation. But if you’re a poor farmer, the loss of one of your animals can mean a lot. That’s why I say almost all bats are innocuous… and really, vampire bats aren’t that bad, either. Get the skinny here from, of all people, Bat Conservation International.