The spring and fall migration periods are receiving increasing attention in the context of songbird conservation. Conditions encountered during active migration, such as variable habitat availability and quality, tall man-made structures, and adverse weather, all can directly affect the survival of migrant birds. Changes in numbers of migrating birds at points along migration routes may be an index to changes in the size of the source populations of these migrants, trends that are related to survival and productivity. For some populations of birds, such as those that occur in remote boreal regions of North America where little or no direct population monitoring is done, monitoring changes in numbers of migrants is the principal tool available for assessing population trends.

Until very recently, migrant passerines have largely been monitored via daytime counts and through capture at banding stations. Because most songbirds migrate at night, and because many variables, like regional and local weather, dictate where, when, and for how long birds stop over following a night’s migration, as well as their detectability or catchability during stopovers, long-term population trends may only become apparent in point count and banding data after many years, and then only when the trends themselves become very pronounced. It would be beneficial if there were a more sensitive means for monitoring populations and detecting negative trends before they threaten the long-term conservation of species.

Many birds emit short audible flight notes during their active nocturnal migrations. If we could use these notes to identify and enumerate the birds flying overhead, then we would have another tool for monitoring populations of migratory birds. Although it has been scientifically studied for more than 50 years, only in the last 5-10 years have our knowledge of bird vocalizations and acoustical and computer technologies come together to make acoustical monitoring a viable tool for population monitoring. A leader in this field is Bill Evans, who pioneered recording technologies and began the painstaking process of identifying the flight notes produced by many species (there is a great deal of information on this at his Web site, www.oldbird.org). Tremendous strides in the development of software for analyzing acoustical data has been accomplished by researchers at Cornell Lab of Ornithology and Cape May Bird Observatory.

Recently, PARC has joined in the effort to advance the techniques and technologies surrounding acoustical monitoring as a population monitoring and songbird conservation tool. Beginning in fall 2003, directional microphones to record flight calls have been set out and run near Powdermill. Currently, three stations are in operation: two at Powdermill and a third atop Laurel Mountain to the east. The microphones are attached to computers which are programmed to record for about twelve hours each night, from sunset to sunrise. These recordings are then analyzed for flight call notes, which are extracted from the nights' data and individually saved by date, time, and frequency. Flight call notes are categorized by frequency and manually sorted into species groups. Analysis of the data then determines the minimum number of individuals, temporal patterns of flight calling, and peak passage times.

A discovery made by PARC’s Assistant Field Ornithology Projects Coordinator, Mike Lanzone, led to the design of an “acoustic cone” which is now being used to obtain the nocturnal flight calls of selected birds banded at Powdermill. This has resulted in a rapid increase in the number of species for which flight calls and variation in these calls are known. Acoustical data collected are archived and linked by band number to the corresponding records in the PARC banding database, which provides information about the age, sex, and energetic condition (fat levels and body mass) of the all recorded birds. Statistical analyses of intra- and inter-specific variation in flight call notes will help facilitate more robust methods for distinguishing species-specific flight notes using advanced software, and information on age, sex, and energetic condition of birds in relation to flight call rates in captivity will aid in the translation of the number of recorded calls overnight into an accurate estimate of the number of individuals of each flight-calling species that passes over a location in migration at night.

Working with colleagues and students from Cornell Lab of Ornithology, Cape May Bird Observatory, and the University of Pittsburgh, which operates the Airglow Observatory atop Laurel Mountain, where one of our recording directional microphones is permanently installed, PARC is playing a role in what we believe are very important advances in the application of bioacoustics to the monitoring of geographically remote breeding populations of songbirds in North America and, eventually, around the world.