Regional declines in Mexican free-tailed bat emergence population quantified by weather radars

Insectivorous bats play key roles in the ecosystem and agroecology by providing essential services for pest control and serving as ecotourism attractions, but information is urgently needed about their response to the changing climate. In Texas, this is especially true, as Mexican free-tailed bats (<i>Tadarida </i><i>brasiliensis</i>) have formed the densest aggregations of mammals anywhere on the planet. However, their population status and phenology in this region are understudied, with most of the knowledge coming from just a few dense colonies. Little is known about the neighboring cave and bridge colonies throughout the south-central Texas region. Nightly, millions of bats emerge from colonies to forage around sunset. These dense aggregations in the aerial space can be captured by weather radar. We quantified the nightly number of Mexican free-tailed bats from four weather surveillances radars in Texas. Analyzing data from 53 consistent colonies over 17 years (2006-2022), we observed a region-wide decline in their emergence activity at an annual rate of 3.37%, with varying rates of decline across individual colonies. Pronounced decreases in overall bat activity levels were also observed annually. Phenological shifts were generally minimal except for a delay observed in the later stages of fall dispersal.<b> </b>This study provides the first comprehensive assessment of multi-colony abundance, population trends, and phenology shifts in Mexican free-tailed bats.