If you're ever down by the Congress Avenue Bridge in Austin, Texas during a summer evening, you may be able to observe a strange phenomenon. At about sundown every night, around 1.5million Mexican free-tailed bats come streaming out from under the bridge, at speeds of up to 100km per hour. They fly up into the sky in black clouds, venturing out for their night-time meal of insects. They can climb to an altitude of about 3300m, which sometimes renders them visible on air traffic radar. When they return to their roosts at sunrise, they will have eaten between 10,000 and 30,000 pounds of insects — thus keeping Austin's mosquito population at a reasonable level. The event is very popular, and is one of Austin's major tourist attractions.

A species of bat common to Mexico and the south-western United States, Mexican free-tailed bats are about 9cm long and weigh about 15g. Though they winter in Mexico, Mexican free-tailed bats come north in the spring, in order to give birth and to take advantage of the thriving insect population in Texas. They make their homes in caves and tunnels and under bridges, so it is little wonder that so many of them have gravitated to the Congress Avenue Bridge. The Congress Avenue bats in fact comprise the largest urban colony of Mexican free-tailed bats in North America.

The nightly event is presided over by Bat Conservation International, which seeks to raise awareness about bats and their protection. Representatives from BCI can be found at the bridge during bat-flying time, handing out newsletters and information about bat conservation.

Bat History

The flight of the bats began in 1980. When the Congress Avenue Bridge, which spans Town Lake¹, was renovated, joints were added under the bridge to allow for the concrete's expansion during warm weather. These joints were, coincidentally, perfectly suited to bats. When this factor was combined with the fact that human invasion was forcing Texas bats out of their native habitats, it resulted in the first of the nightly bat flights.

At first, Austin residents were terrified by the clouds of bats which invaded the air. A contemporary newspaper headline reported 'Mass fear in the air as bats invade Austin,' and by 1984, when the bridge's bat population peaked, a public health crisis was declared². Austin residents, in a state of panic, demanded that the bats be removed from the city's centre.

Luckily for the bats, though, Bat Conservation International, a Milwaukee-based non-profit organisation, took interest in the situation in 1986. Moving their headquarters from Wisconsin to Texas, BCI undertook to educate the public about the bats. Now, from Thursday to Sunday, in the summer months, BCI, in cooperation with the Austin American-Statesman³, offers educational activities for children and 'bat interpreters' at the bridge, encouraging awareness of Texas bats.

It should be noted that, although no instances of bat-transmitted diseases have been reported to date, some bats do carry rabies, which can be passed on to humans. Bats should never be touched, no matter how enticing this may seem. If a child is found to have been in contact with a bat, they should be tested for rabies prophylaxis. The Texas Department of Health and BCI carry very clear warnings to this effect.

How to Get There

If you happen to be in Austin, getting to the Congress Avenue Bridge is quite simple. Congress Avenue is the main north-south street in central Austin and can be reached from both I-35 and the MoPac Expressway. On summer evenings, the Austin American-Statesman buildings, which are just to the south of the bridge, offer free parking for bat-watchers⁴. From there, it is a simple two-minute walk to the bridge, or to the Hike and Bike Trail which runs along both banks of the river and which offers the best views of the bats.

If you are already in the middle of town, the bridge is no more than a ten to 15 minute walk from most of downtown Austin's attractions, including the Texas State Capitol, Sixth Street, Antone's Blues Club, the Warehouse District, and Threadgill's South. The Capitol Building is also the nexus for dozens of bus routes operated by Austin's public transport authority Capital Metro, a number of which actually run across the Congress Avenue bridge.

Depending on the time of sunset, the predicted bat emergence time can vary, so you might wish to call the Bat Hotline to find out the time. The number can be found on BCI's website. You may also want to bring chairs or a blanket to sit on, as the wait can get rather tedious if one is just sitting on the ground.