The South Congress Bridge in Austin houses the largest urban population of bats in North America: an incredible 1.5 million bats sharing one bridge. This is a small colony, however, compared with what’s found in nature: At Bracken Cave, north of San Antonio, the colony numbers 20 million.
A mother bat returning at dawn to roost can find the baby she left behind simply by its distinctive cry and its distinctive smell. Mothers, if you’ve ever lost track of your child in a department store and panicked, try to imagine doing that every night… except you have to pick your child out of a crowd of millions, in the dark, when all the children look the same, and they’re all crying for their mommies at once. Every mother thinks their child is unique, but the Free-Tailed Bat knows just how unique, down to the slightest inflection of voice and chemical molecule of scent.
But the truly remarkable thing about Mexican free-tailed bats is how they behave like a storm front.
For a very long time, nobody knew where Austin’s 1.5 million bats went at night. They seem to fly off along the Colorado River and disappear into the livid dusk. It turns out that the colony is part of a supercolony with caves all over South Texas, which rises up to 10,000 feet into the air and forms into a living cloud.
Here they are on Doppler Radar, with the Austin bats joining late in the game. What is this voracious cloud after? That stormfront of cornworm moths flying in from the Southeast, heading straight for Texas’ agricultural center. It turns out that every night of summer, an epic war is waged at high altitude above Texas, with rapacious locusts and moths trying to reach the breadbasket to devour its crops, and an equally hungry super-cloud of bats trying to stop them. Cue “Ride of the Valkyries,” with barely audible squeaking.
This is the power of numbers in action. The colonies must be enormous, or the entire ecosystem is laid to waste. The only hand either army can have over another is to outbreed or outclimb another, so the battle has evolved to take place over the moon, and so thick it can be seen from space. The bugs and the bats crash into each other in the billions, like the warm and cold air that makes tornadoes. Stomachs are filled, mothwings fall like snow. And in the end, every mother goes home to the multitudes, to nurse her one and only.