California’s Central Valley – Mexican free-tailed bats deliver essential pest control for the region’s farms, yet rising summer temperatures have triggered repeated die-offs at key roosts. Researchers are now tracking how these animals manage body heat during extreme conditions to guide future protection efforts. The work draws support from the National Wildlife Federation and highlights stakes for both wildlife and agriculture.
Heat-Linked Die-Offs Raise Alarm
Multiple mortality events have struck maternity roosts across the Central Valley in recent years. In June 2022, more than 200 Mexican free-tailed bats were found dead beneath a roost in Modesto. Between June 2022 and August 2025, at least 100 bats died in separate incidents beneath a bridge in Butte City. In July 2024, more than 1,000 pups perished beneath a roost in Bakersfield, followed by several hundred additional deaths near Fresno in September. ([1])
Each incident occurred during peak summer heat. Maternity roosts serve as critical sites where females give birth and raise young before the bats migrate south. The largest such colony in California holds more than a quarter million individuals. These losses have prompted questions about how well the species can withstand a warming climate.
Practical Stakes for Farms and Food
Mexican free-tailed bats forage high above fields and orchards, intercepting insects before they can lay eggs on crops. This service benefits rice, vegetables, fruit, and nut production throughout the Central Valley, one of the nation’s leading agricultural areas. Across the United States, bats deliver billions of dollars in natural pest control each year.
Protecting these bats therefore carries direct implications for farmers and the food supply. The species ranks as the most common bat in the Central Valley and the world’s fastest mammal. Continued mortality at roosts could reduce this free service at a time when climate pressures are intensifying.
Tracking Body Temperature in the Field
Researchers at California State University, Sacramento, launched a study after witnessing one of the die-offs. They capture bats emerging from a maternity roost and attach temperature-sensitive transmitters to monitor body temperature during late-summer heat. Loggers placed inside the roosts record ambient conditions for comparison.
The project receives funding through the Wild Kingdom College and University Grant Program, a partnership involving Mutual of Omaha’s Wild Kingdom and the National Wildlife Federation. Students assist with capture, processing, and release, gaining direct field experience. The data will clarify how these bats respond physiologically to extreme heat and whether current roost conditions remain viable.
Outreach and Next Steps
Alongside the fieldwork, the team conducts public events, including presentations at the California State Capitol. These sessions introduce legislators and residents to the bats’ role in the ecosystem. The dual focus on research and education aims to build broader support for conservation measures.
Findings from the transmitters and loggers are expected to inform targeted actions, such as roost modifications or habitat protections. As summers grow hotter, understanding these responses becomes essential for maintaining both bat populations and the agricultural benefits they provide. The effort underscores the link between wildlife health and the stability of food production in a changing climate.
