Bats and Angel Rings: Quantifying Flight Behavior and Colony Dynamics using NEXRAD Doppler Radar Data
Jason W. Horn
Thomas H. Kunz
Last modified: September 5, 2007
In 1957, the U.S. government built the first national network of radar systems for the purpose of weather surveillance. Radar operators soon noticed echoes on their displays that did not move or behave like weather patterns, including expanding ring-like shapes they sometimes called angel rings. These echoes were caused by high-flying insects, bird migrations, and large colonies of bats emerging from their roosts to feed. Modern WSR-88D weather radar stations (also known as Next Generation Radar or NEXRAD) now provide data from which detailed images of bat emergences can be produced and flight behavior of groups of bats can be investigated. Large cave and bridge colonies of Brazilian free-tailed bats (Tadarida brasiliensis) in south-central Texas are clearly imaged by three local WRD88-D Doppler RADAR stations. In this study, we used Doppler RADAR reflectivity and radial velocity images to examine colony size, direction of movement, speed of dispersion, and altitude gradients of bats from these colonies during the hours after evening emergence. Base reflectivity clear-air mode level II NEXRAD images were geo-referenced and compiled in a GIS along with colony locations and landscape features. Temporal sequences of images were masked at selected colony locations and spatial means and total reflectivity were calculated. These measures, combined with data from radial velocity images, were used to compute directional vectors and compare emergence behavior using circular statistical models. Relative colony size and inter-annual changes were also measured. Our results indicate that multiple factors play a role in the nightly flight and dispersal behavior, and that NEXRAD data can be a useful tool for monitoring the nightly behavior of large bat colonies and seasonal changes in these populations. Understanding the distribution of a large regional bat population on a landscape scale has important implications for agricultural pest management and conservation efforts.