- Christine Ayala American-Statesman Staff
Recent cool temperatures, rainy weather and an abundance of insects make for ideal conditions for the Mexican free-tail bats living under the Ann Richards Congress Avenue bridge, but that is not so ideal for onlookers hoping to catch a glimpse of the bat swarm. The bats are taking shorter hunting trips and leaving later when viewers cannot as easily see them.
The bat population, which at 1.5 million is the largest colony in North America, is healthier and more stable than it has been in recent years because of the cooler weather, said Dianne Odegard, Bat Conservation International’s public education and training coordinator.
“The bats are well-fed, since we’ve had quite a bit of rain, there are a lot of insects out for them to eat,” she said. “They don’t really need to come out early. It’s riskier for them to come out when it is still daylight because they are in danger of predation by hawks, and they’re also less likely if there are a whole bunch of people shining lights on them.”
Instead of seeing a hungry surge of bats all at once, like in recent years when the dry, hot summer conditions made finding bugs more difficult, the bats that are visible are coming and going in a thin and unhurried stream.
Mexican free-tails eat mainly moths and beetles, but they will also eat flies and other easily caught insects. Unlike other bat species, they will adapt their feeding schedule from season to season based on their need for food and to avoid competition, said Liz Braun de Torrez, a doctoral bat researcher at Boston University.
“Bats ideally come out after dark, but, depending on the weather conditions, the time of emergence changes drastically,” Braun de Torrez said. “This is good news for the bats — though not so good for tourism. There’s so many of them and so much competition for insects, so these are ideal conditions when they don’t have to work as hard.”
The past two summers the bats emerged during the 8 o’clock hour before or at sunset, said Shannon Shaddock, general manager for Capital Cruises, which offers nightly bat-watching boat tours.
“Right now, we are seeing them right around 9 p.m.,” Shaddock said. “In the immediate past, when we’ve had no rain whatsoever, we’ve had great viewing much earlier. They are having a more typical season this time.”
June is also the birthing month for the bat colony, which is another reason the bats are emerging later than some might expect. A nursing mother must eat her own weight in insects daily to produce enough milk to feed her pup, which eventually drives her out in search of food. Each mother will only feed her own pup and needs to keep track of it under the dark and noisy bridge, full of other newborns.
“They spend a period of time together, getting used to each other’s scent and sounds, their call, so they can identify each other,” Odegard said. “So they will leave later during the birthing time to recognize their pup.”
Neighboring bat colonies are having equally offbeat schedules. The McNeil bridge bats in Round Rock are behaving erratically with widely varying departures from day to day because of rain patterns. Some nights they leave as early as half an hour before sunset and other nights long after dark. The Bracken Cave bats near San Antonio are emerging earlier, around 7:45 p.m. because they need to travel farther to reach agricultural areas or water ways for insects.
Despite the cool weather and birthing season, Shaddock said viewers can still spot the Congress Avenue bats if they know where to be.
“It is more a matter of where you are looking from than when they come out,” she said. “They are more difficult to see if you’re in the wrong spot; you can’t always line up on the north shore and still be able to see them. They pretty much always emerge from the south shore, and you should see them there.”