The historic drought in Texas is changing the behavior of the stars of one of the state's natural summer marvels, raising fears the spectacle next year may be less spectacular.
A depleting insect population has forced millions of bats around Texas to emerge before nightfall for food runs, making them more susceptible to natural predators. Some experts have already noticed fewer bats emerging from caves and have seen evidence that more infant bats are showing up dead, hinting at a looming population decline.
"The drought makes for good bat viewing but is hard on the bats," says Fran Hutchins, coordinator at Bracken Cave, home to the world's largest concentration of bats, about an hour's drive northeast of San Antonio.
The cave holds some 20 million Mexican free-tailed bats that emerge for food runs every evening from March into November in numbers so great they show up on weather radar. The major difference this year is they're emerging much earlier, as much as two hours before darkness, because they need to travel farther to find the drought-depleted insect populations such as moths that infest corn and cotton plants.
"Last year, when it was really wet and there were good crops and good for pests, the bats were emerging just about dark," said James Eggers, education director at the Bat Conservation International, which manages Bracken Cave. "That's when they prefer to emerge because they can avoid all different kinds of predators."
Mexican free-tailed bats, the most common species in Texas, are targeted by hawks and falcons that dive into the swarm emerging from the black mouth of the cave, an ancient collapsed sinkhole where tons of excrement gives off an ammonia-like scent. They're also hunted by raccoons, owls and snakes that sometimes dangle from the top of the cave entrance, snatching bats as they come out in a whirring, corkscrew-like funnel that darkens the sky. Eggers said the declining insect population will also affect the bats' survival rate, though it will be difficult to determine the severity of the problem until 2012.
"We can stipulate that more than most years, the old, the infirmed and the young are suffering more, so bat populations will be lower next year," Eggers said.
The dark brown or gray bats are about 15 grams each -- the weight of two 25-cent coins -- with wingspans of 8 to 10 inches. They are among the slowest reproducing mammals, producing each year a single pup, whose life is in jeopardy if the mother bat can't provide sufficient food.
These factors have experts concerned about other colonies across Texas where the bats are also beginning their food runs before nightfall. This includes the 1.5 million bats in Austin -- the world's largest urban bat population -- which entertain people with nightly departures from under a city bridge.
Activity at another well-known Texas bat site may be foreshadowing next year.
Nyta Brown, who manages the Old Tunnel Wildlife Management Area near Fredericksburg, said while her bat emergences are earlier this summer, the 3 million bats that normally inhabit the abandoned train tunnel is down considerably. Nightly shows that should be 40 minutes to an hour at this time of the year have dwindled to six to 10 minutes.
"Usually the population at Old Tunnel almost doubles during the month of August as the juveniles have begun to fly," she said.
"There is the possibility that many of the pups did not survive this season," Brown said.
In Houston, Texas Parks and Wildlife Department urban biologist Diana Foss has seen some slight behavior changes in bats that live under bridges along bayous near downtown but suggested it's more likely due to record heat. The largest colony has about 300,000 bats.
"There seems to be enough insects, but whether it's the type of insects they're looking for, we haven't done any study," she said.
What Foss has noticed is an increase in the number of baby bats falling to the ground and dying, potentially meaning a smaller presence next year.
"If adults aren't able to get enough food, babies don't," she said.
Texas A&M biologist Mike Smotherman isn't so sure the behavioral changes are dire.
"These things are everywhere," he said. "These are fantastically mobile animals designed to fly fast and go far distances."
Smotherman said his studies show if bats don't like the food or water, they just move somewhere else.
"Given a choice, they do have preferred foraging grounds, but they make annual migrations over thousands of miles and it's hard to imagine they're really stuck on one path," he said. "My guess is they're flexible enough and adaptable enough they can just deal with it."