Bats

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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.”

TAMIR KALIFA / AMER­I­CAN-STATESMAN

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.

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