Mexican free-tailed bats provide enormous economic and agricultural value to the Central Valley of California. Monitoring these populations is crucial to understanding their role in pest control services and their overall health. Additionally, the results of conservation efforts can be measured by monitoring specific bat populations. We periodically counted roosting bats under the Franklin Boulevard Bridge and Causeway from the summer of 2004 to the summer of 2025. Bats were excluded from a bridge in 2000 with about 40,000 bats. For the ensuing 4 years between 500 and 2,000 bats roosted among 7 bat condominiums. Sixty days after the replacement Franklin Blvd Bridge was completed, 15,600 bats moved into the bridge’s roosting habitat. A year later the population had increased to over 20,000. The population continued to increase by an average of 8,500 bats/year until 2019 at 141,000 bats. The two summer counts since then suggest that the population has stabilized at about at about 140,000. Our data suggest that new habitat enabled the bat population to increase for about 20 years. These data also demonstrate that replacement roosting habitat can be very successful.

Dr. Dave Johnston is an Adjunct Associate Wildlife Ecologist and Bat Biologist at H. T. Harvey & Associates. Dave is a vertebrate ecologist who specializes in the foraging ecology and conservation biology of bats. He has studied bats for over 30 years and for the past 15 years he has focused on renewable energy and transportation projects in California and Hawaii. He also has ongoing research projects involving the foraging ecology of bats in California, Mexico, Belize, and more recently, in Costa Rica where he lived for one year. Dr. Johnston is a hobby winemaker who started making wine as a student at CalPoly, San Luis Obispo.