In 2002, the US Geological Survey began a series of bat inventory studies in San Diego County funded by several federal, state, and local agencies including the National Park Service, United States Forest Service, California Department of Fish and Game, and the County of San Diego. Four study areas are being surveyed including the Cabrillo National Monument, Descanso district of the Cleveland National Forest, Santa Ysabel Ranch Preserve, and the Multi-Species Planning Area (MSCP) of eastern San Diego County.

Several survey methods are being utilized to conduct these bat inventories including 1) use of mist-nets to capture bats at foraging and roosting sites, 2) use of hand-nets to capture bats at day and night roosts, 3) use of acoustic techniques including Anabat bat detectors and audible listening to detect bats at foraging and roosting sites, and 4) use of artificial lights to visually inspect for bats at roost sites and to visually observe foraging bats. Most of the survey effort has occurred from May through September, however, surveys are being conducted year-round with a reduced but consistent, almost weekly, effort throughout the fall, winter, and spring.

As of February 1, 2003 a total of 17 of the 23 bat species known from San Diego County have been detected during the USGS bat inventory studies. Thirteen of these 17 species have been documented by capture at roosting and/or foraging sites, with the remaining four species having been detected only acoustically.