Participants at the National Avian-Wind Power Planning Workshop III recognized that there is inadequate knowledge concerning the effect of wind turbines on night-migrating birds. The emerging technique of acoustic monitoring of avian night flight calls, as the only means for acquiring species-specific information about birds in active night migration, is a method uniquely suited to help fill this void. Sensitive microphones aimed at the night sky are used in recording the vocalizations of night-migrating birds. About 200 species of North American birds are known to give calls during night migration, with roughly 150 of these being distinctive enough to identify with certainty. Others are currently lumped into a number of similar-call complexes. The calls by individual species within these groups are not reliably distinguishable from one another at present (Evans and Rosenberg 1999). On a good migration night east of the Rocky Mountains, thousands of calls may be recorded from a single monitoring station. In the west, calling rates are believed to be lower, though little acoustic work has been conducted there. Nocturnal flight call monitoring has evolved slowly during the 20th century, limited by the difficulty in identifying many of the cryptic night flight calls from passerines, and by the challenge of processing the large quantities of data generated. Recent progress on both of these fronts has been aided by advances in electronics and computers.
Three applications of acoustic monitoring for assessing and minimizing the impact of wind turbines on night-migrating birds are discussed in this paper. Two of these applications were carried out in a study for Nebraska Public Power District (NPPD) at a proposed wind turbine site during the fall 1996 and spring 1997 migration periods. A third application was tested experimentally in fall 1994 at an existing wind turbine site in northern New York State operated by Niagara Mohawk Power Corporation (NIMO).