It's almost always bat season in Austin.
Globs of residents and visitors drape themselves on the Congress Avenue Bridge every night between March and early November to watch the roughly 1.5 million Mexican free-tailed bats fly from beneath their feet.
But bat season is getting even longer, the Austin American Statesman reported.
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In the past three to five years, thousands of bats in the world's largest urban bat colony have been extending their stay in town because Austin's average temperatures have gotten warmer or because the animals are sick, said Sarah Whitson, a wildlife officer for Austin Animal Protection.
Temperatures in the 30s and 40s help the bats know it's time to head south where they can stay warm for the winter, Whitson said.
"The biggest reason they're migrating is food supply, so if there continues to be warmer weather, there continues to be more insects for them to eat -- so that will keep them around longer or bring them back sooner," said Fran Hutchins, director of Bracken Cave Preserve.
Bats eat crickets and other insects, including agricultural pests like beetles and moths. Usually that food supply dwindles with cold weather, but if it stays warm, more insects are around to eat, Whitson said.
Hordes of crickets overran Austin in October -- providing food for bats and other predators like snakes and lizards -- as Central Texas continued to experience summer-like temperatures in the 90s several weeks into fall.
Ross Winton, a biologist specializing in bugs and other invertebrates for the Texas Parks and Wildlife Department, said cold and moist weather work as a trigger for crickets to mate, lay eggs and die.
Cold weather also works as a trigger to leave for bats, who like temperatures in the 70s, 80s and 90s, Whitson said.
In a recent week, Austin temperatures hit a high of 85 degrees -- about 10 degrees warmer than normal, according to the National Weather Service -- and that Sunday's high was 76. But Central Texas that Monday was preparing for a hard freeze overnight into the following day.
"Our weather here in Texas is very random. We can have all four seasons in one day," Whitson said.
September was the hottest ever recorded in Austin and October had 11 days with temperatures in the 90s, according to the weather service.
Bats confused by the weather may stay through December and January -- and if they stay that long, they won't migrate, Whitson said. Bats that are sick with rabies or other illnesses also stay behind, she said.
"As climate is changing and summers start to last longer and winter gets shorter, we might see over time that bats may not migrate at all," she said.
But the bats staying longer is not necessarily a bad thing if the food supply is good and temperatures are warm, Whitson and Hutchins said.
Roosts under the Congress Avenue bridge have helped keep bats warm when temperatures start to drop. The spaces are 16 inches deep and 1 inch wide and retain heat for the bats, Whitson said.
The change in the bats' migration could become a concern if the food supply is not there, Hutchins said.
He said crops could be affected by severe drought, which would then affect how many insects are around to eat the crops. Then the bats would have less to eat.
Bat Conservation International, a nonprofit dedicated to bat education and protection, estimates that the suppression of agricultural pests by bats is worth about $74 an acre in Texas.
Travis, Williamson, Bastrop, Hays and Caldwell counties are all experiencing a moderate drought as of Thursday, according to the U.S. Drought Monitor. During a moderate drought, dry land crops can be stunted, wildfire frequency increases, stock tanks, creeks and streams are low and cattle sales begin earlier, according to the U.S. Drought Monitor.
Bats that do migrate during the winter months are coming back earlier than before.
Typically, bat season in Central Texas starts in March, Whitson said. But some bats have been recorded coming back to town in January and February, she said. Forty requests via 311 calls were made in February by Austin residents who needed help removing bats that were indoors or on the ground.
A total of 23 bats were picked up in February, according to Austin Animal Protection data.
Hutchins was not sure what is triggering the bats to come back sooner than in the past.
"They have a really good spot down there at the Congress bridge," Whitson said.