Bracken Cave bats moving north from Mexico two weeks earlier, more overwintering in Texas

A changing climate has prompted Mexican free-tailed bats at the largest bat cave on the planet to advance their migration north from Mexico by two weeks in the spring, a new study suggests.

Bats are arriving in Texas from Mexico two weeks ahead of schedule.

For hundreds, perhaps thousands of years, millions of bats have arrived at Bracken Cave on San Antonio’s north side in mid-late March. But now the bats are arriving two weeks or more earlier, a study of 23 years of weather radar data indicates. The research, conducted by atmospheric scientist Phillip Stepanian at the University of Oklahoma and Charlotte Wainwright of Rothamsted Research, was published in Global Change Ecology last month.

The study confirms what local naturalists and scientists have been noticing for years: spring is arriving earlier.

Weather forecasters often notice bats on radar. Courtesy graphic

“Over the last five or six years, we’ve seen the bats returning earlier, coming back around February 21st,” said Fran Hutchins, Director of Bracken Cave Preserve, which is overseen by Bat Conservation International (BCI). Hutchins said BCI is observing similar patterns at other preserves, including colonies at bridges, tunnels and other caves around Texas.

Stepanian, lead author of the study, reviewed 23 years of local weather radar data. “There’s lots of stuff in the sky that weathermen don’t care about,” he said. Bats, birds, even butterflies show up when they move en masse through the skies. “Their bodies are pretty much really really really big rain drops,” said Stepanian by phone, “and radar picks up masses of water.”

Stepanian explained that In 2013, the National Weather Service (NWS) upgraded the entire radar system nationwide. The improvements led to greater abilities for weather forecasters to distinguish between weather and biology. They learned quickly that if the same “cloud” appears each evening at dusk north of San Antonio, it’s not a thunderstorm, it’s bats.

Mexican free-tailed bats take flight from Bracken Cave in San Antonio. Photo by Phillip Stepanian

Last summer, a storm of Painted Lady butterflies migrating north through the central U.S. caused a national stir when their 70-mile-wide biomass tracked around Denver. The NWS issued a bulletin explaining the situation and detailed 17 other non weather phenomena that can be detected by radar–flying ants, birds, and beetles as well as chair lifts, wildfires, even severe wind.

Stepanian noted that the uploading of historical data to the web that resulted from  the 2013 radar upgrade made decades of information accessible. Improved computing power facilitated its analysis.

He expects the trend to continue. Warmer temperatures will cause moths and other insects to hatch earlier, and the bats will follow the food. Already, more bats are not even leaving the cave, making San Antonio their winter home. Stepanian’s research shows that two decades ago, one percent of the millions of bats at Bracken remained when migration season arrived in the fall. Now, 3.5 percent of the bats choose to stay here as winter Texans.

Dr. Phillip Stepanian

“We have an overwintering population, and that has definitely increased over the years,” said BCI’s Hutchins. “We have warm enough nights that they can come out and forage for food, and we have plenty of insects.” Hutchins noted that despite an earlier arrival, the bats don’t seem to be leaving any later–unless we get a good cold snap, which drives them south.

Could overwintering in the cave have a down side for the bats?

“We just don’t know,” said Stepanian. Some ecologists have expressed concerns that overwintering in their summer homes could cause health problems. The spaces might not have a chance to air out and could become a hotbed for disease or parasites. Also, the gene pool of a migratory population is more diverse, thus more healthy.

Bats from the Bracken Cave bat colony number 15 – 20 million bats in their late summer peak and constitute what is widely considered the largest collection of living mammals on earth. At their maximum population, they consume 140 tons of insects per night. The bats perform an estimated $22 billion per year in valuable ecosystem services, ridding their territories of destructive agricultural pests like corn earworm and fall army worm. They also eat massive amounts of pesky mosquitoes which can carry viruses.

The implications of the changes in their migratory patterns for agriculture and human health will be the subject of future studies.

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Bats, Blooms, Butterflies and Moths–Everyone is Early this year

Our wet and mild winter has migratory creatures and seasonal blooms arriving in Central and South Texas early this year.  According to biologists and naturalists, we’re running seven – 10 days ahead of schedule.

Monarch butterflies, which typically start showing up in Texas en masse in late March, have been spotted regularly since early in the month.  Over at Bracken Bat Cave, maternal bats who overwinter in Mexico also arrived ahead of schedule.

Caterpillar on Bluebonnet

Caterpillars and bluebonnets--both early this year

“This year they were 10 days early,” says Fran Hutchins, Bracken Bat Cave coordinator and a Texas Master Naturalist.  Hutchins adds that the insect eating mammals began showing up in waves around February 21.  “There hasn’t been a lot of research on specific dates of their comings and goings,” says Hutchins, explaining that he inadvertently noticed the increase in bat population while completing an overwintering survey at the Cave. Congress Ave. Bridge bats returned early this year.

Congress Ave. Bridge bats were early this year

Bats returned early this year

The pattern holds for wildflowers and birds. “It’s definitely early,” says Andrea Delong-Amaya, director of Horticulture at the Ladybird Johnson Wildflower Center. “But in terms of what’s normal, it’s hard to say.  It just hasn’t been as cold.”  Reports of the Golden Cheeked Warbler, our local endangered songbird, arriving a bit early have made the rounds at the San Antonio Audubon Society, according to Martin Reid, an avid birder and environmental consultant.

“It’s a mixed bag: some of our resident birds are showing signs of breeding activity slightly earlier than usual–probably related to rain,”  he explains.  “But it doesn’t seem to have much effect on the wintering birds.”

Is it the warmth or the wet that drives the timing?   Depends on who you ask.

“On average I think the Weather Service is better than us insect experts at predicting the future,” says Dr. Mike Merchant, writing on Texas Agrilife Extension’s delightful Insects in the City blog. “But I still don’t put too much stock in long-term weather forecasts.”   Dr. Merchant chronicles the early arrival of Armyworms to North Texas in a recent post.   The gregarious grass eaters get their name from reproducing in droves and marching across prairies in soldier-like formations.

Armyworm Moth in Lawn

Armyworm Moths have arrived early to North Texas -- photo Texas Agrilife Extension

Matt Reidy points to the weather.  “Pepper weeds, bluebonnets, prickly poppies–those are all early.  Not because of temperatures, but because of rain,”  says Reidy,  Texas Parks and Wildlife Biologist.  “When you get the moisture, that’s what determines what you’re gonna get when.”

Many peg climate change for the advance of the season.   Interestingly, February 2012’s average high temperature was about the same as–actually .07 degree less than–the historic average of 66 degrees in San Antonio.   Yet, the average LOW temperature for the month was 4.5 degrees higher than average.

Minimum temperatures are especially impactful to seed germination and plant growth.  Seeds and plants require a certain soil temperature in which to germinate and thrive.  Savvy gardeners know to put a heating pad under setting seeds to expedite sprouting. Higher average minimum temperatures translate into faster growth, and an earlier season.

At the Children’s Garden at the San Antonio Botanical Garden, volunteers planted tomatoes the first week in March–“weeks early,” according to David Rodriguez, Horticulture specialist for the Texas Agrilife Extension.  Those tomatoes will likely be ready the first week in May.   “Everything’s off,” says Rodriguez, referring to Nature’s unpredictable timing.

Earlier this year, the USDA announced changes in plant hardiness zones, moving parts of San Antonio into the same planting zone as Houston and Corpus Christi. Some San Antonio zip codes moved from zone 8b, with annual lows of 15 – 20 degrees, to zone 9a, with annual lows of 20-25 degrees.

The redrawn maps (plug in your zip code and find out your zone here)  seem to be telling us something that birds, butterflies and bats have known for awhile: it’s just not as cold as it used to be.

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