exactly where they all go, according to Danielle O’Neil, the public engagement intern for Bat Conservation International. But before getting into the particulars, a bit of general information might help.
‘The battiest state in the country’
During part of the year, the Congress Avenue Bridge is home to the world’s largest urban bat colony. About 750,000 Mexican free-tailed bats begin migrating in March from central Mexico to the crevices beneath the bridge. Each bat weighs about half an ounce and can fit in the palms of American-Statesman reporters who periodically have to catch one that has sneaked into the newsroom.
The Congress Avenue Bridge colony has only female adults, according to Bat Conservation International.
A good time to see them is during gestation in April and May, when they’re particularly hungry. In June, most of the bats give birth to a pup (baby bat), roughly doubling the population.
The bat conservation group notes that each pup is about one-third the weight of its mother — the equivalent of a human giving birth to a 40-pound child.
Most mothers stash their pups on the north — “nursery” — end of the bridge. Up to 500 pups cluster per square foot, yet the mothers manage to find their pups each night.
The mothers nurse for about five weeks, until the pups are ready to fly and hunt on their own. Adult male bats cluster in other parts of town, often roosting on the sides of buildings.
Kayakers and party boats passing under the bridge each day can hear the “colony chatter,” which is more or less constant, and possibly sniff the pungent aroma of the bats.
The sight of the bats taking flight draws an estimated 100,000 people each year to the bridge. So many bats are in the colony that it might take three hours to clear out of the bridge to hunt. The bats eat 10,000 to 20,000 pounds of insects each night, including agricultural pests, according to the bat conservation group’s estimates.
Austin now has numerous monuments recognizing the bat population, which, in the early 1980s, some petitioned the city to rid itself of, fearing diseases such as rabies. (The bats pose no threat as long as no one tries to pick up any that have fallen to the ground, according to the city health department.) Count former Mayor Lee Leffingwell among the bats’ fans.
Austin is one of a dozen major bat-watching sites around the state, according to the Texas Parks and Wildlife Department, which calls Texas “the battiest state in the country.”
The Austin colony tends to head back to Mexico around the start of November.
Many variables decide destination
Hollis, an AARP spokesman who is married to American-Statesman reporter Lilly Rockwell, wondered where exactly the bats go at night, once they’re out of sight. This is the reply from O’Neil, the bat conservation spokeswoman:
“Really, we are not 100 percent sure where (they) go at night as they do not all stay together after emergence. They hunt the shorelines and tree canopy along the river, and then head mostly east to the farmlands to feed on their favorite prey items: corn earworm moths (also called cotton bollworm moths).
“They do not always go to the same location or spend the same amount of time out at night. It is dependent on many differing factors such as weather, if they are nursing their pup, if they are a newly weaned pup and if they find enough food quickly or need to hunt for a longer period of time.”
BCI compiles its information from farmers and from scientific studies about the bats. Researchers often look at other environmental factors to get a better idea of where the bats go and what they’re eating. For instance, scientists have refined what they know about the bat ranges by looking at how they affect the seasonal movement of certain insects.
Hollis was also curious about how high the bats soar. According to O’Neil: “Mexican free-tailed bats can fly quite high. Some have been found up to 10,000 feet chasing moths and other prey, but for the most part they stay around 600 to 3,000 feet off the ground.”
Hollis said he’s heard various theories about the bats and where they go.
“The explanations are really fascinating,” he said. “I had no idea that so many variables are involved.”
Part of the colony of Mexican free-tailed bats flies past Dan Herron as he photographs their exit from the Ann W. Richards Congress Avenue Bridge on Friday. It can take them up to three hours to clear out.