Abstract
This paper discusses an emerging methodology that uses electronic technology to monitor vocalizations of night-migrating birds. On a good migration night in eastern North America, thousands of call notes may be recorded from a single ground-based, audio-recording station, and an array of recording stations across a region may serve as a "recording net" to monitor a broad front of migration. Data from pilot studies in Florida, Texas, New York, and British Columbia illustrate the potential of this technique to gather information that cannot be gathered by more conventional methods, such as mist-netting or diurnal counts. For example, the Texas station detected a major migration of grassland sparrows, and a station in British Columbia detected hundreds of Swainson's Thrushes; both phenomena were not detected with ground monitoring efforts. Night-flight calls of 35 species of migrant landbirds have been identified by spectrographic matching with diurnal calls recorded from known-identity individuals; call types of another 31 species are known, but are not yet distinguishable from other similar calls in several species complexes. Efforts to use signal-processing technology to automate the recording, detection, and identification of night-flight calls are currently under way at the Cornell Lab of Ornithology. Automated monitoring of night-flight calls will soon provide information on migration routes, timing, and relative migration density for many species of birds. Such information has application for conservation planning and management, as well as for assessing population trends.