Analyzing NEXRAD doppler radar images to assess nightly dispersal patterns and population trends in Brazilian free-tailed bats (Tadarida brasiliensis)

Operators of early weather-surveillance radars often observedechoes on their displays that did not behave like weather pattern,including expanding ring-like shapes they called angels. Theseechoes were caused by high-flying insects, migrating birds,and large colonies of bats emerging from roosts to feed. Modernweather-surveillance radar stations in the United States (NEXt-generationRADar or NEXRAD) provide detailed images that clearly show eveningbat emergences from large colonies. These images can be usedto investigate the flight behavior of groups of bats and populationtrends in large colonies of Brazilian free-tailed bats (Tadaridabrasiliensis) in south-central Texas which are clearly imagedby local NEXRAD radar stations. In this study, we used radarreflectivity data from the New Braunfels, Texas NEXRAD stationto examine relative colony size, direction of movement, speedof dispersion, and altitude gradients of bats from these coloniesfollowing evening emergence. Base reflectivity clear-air-modeLevel-II images were geo-referenced and compiled in a GIS alongwith locations of colonies and features on the landscape. Temporalsequences of images were filtered for the activity of bats,and from this, the relative size of bat colonies, and the speedand heading of bat emergences were calculated. Our results indicatecyclical changes in colony size from year to year and that initialheadings taken by bats during emergence flights are highly directional.We found that NEXRAD data can be an effective tool for monitoringthe nightly behavior and seasonal changes in these large colonies.Understanding the distribution of a large regional bat populationon a landscape scale has important implications for agriculturalpest management and conservation efforts.