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Monitoring nocturnal flight calls


 For resources and tools for building a home nocturnal flight call monitoring station, visit the Old Bird website.



A short history of nocturnal flight call monitoring

        On September 14, 1896 on a small hill outside of Madison, Wisconsin, Orin Libby counted 3600 calls from night-migrating birds in five hours of listening (Libby 1899). This count is the first published record of an attempt to quantify the night flight call phenomenon of North America's avifauna. In the 50 years after Libby's report, only two other counts of nocturnal flight calls were reported, those by Paul Howes (1914) and the extraordinary work by Stanley Ball (1952). Both documented the nocturnal migration of thrushes. Ball's work was the first to use nocturnal flight call monitoring to produce data on the migration timing of species in a region. 

        Beginning in the 1950s, regular nocturnal flight call counts were reported from regions throughout eastern North America in the Audubon Field Notes. Most of the reports were tallies of call totals or of interpreted numbers of Catharus thrushes passing over during a portion of an evening. One outstanding exception is the long-term effort by Ron Weir who has been counting thrush calls in fall migration near Kinston, Ontario for more than 25 years (see American Birds, Lake Ontario region, fall reports).

        In the 1950s technological developments enabled the first audio recordings of nocturnal bird migration to be made. Bill Gunn recorded some short periods of calling from atop of the Imperial Oil building in downtown Toronto in late September of 1957. At about the same time, the team of Richard Graber and Bill Cochran were beginning their pioneering nocturnal flight call study in central Illinois. Some recordings of both of these efforts are archived at the Macauley Library of Natural Sounds at Cornell University.

        Graber and Cochran were working through the Illinois Natural History Survey. Cochran was an electrical engineer and designed the recording setup pictured inside this CD-ROM's jacket. He was able to configure a reel-to-reel tape recorder to record for two hours and to automatically record 10-minutes out of every hour. By recording 10-minutes out of every hour all night long, the device was able to sample night flight calling through the course of an evening. This allowed the researchers to sleep while the machine automatically recorded sounds in the night sky. They would later listen to the recordings and log any bird calls. The method allowed them to study the quantity, and to some degree the species composition, of calling during the course of a night, through a migration season, and in different weather conditions. Like Ball, they documented the seasonal timing of migration for a number of species and their work laid the conceptual development for machine-based surveying of nocturnal migration (see Graber and Cochran 1959 & 1960).

        Graber, in his classic 1968 paper, discussed the potential of acoustic monitoring but pointed out several major impediments. One was the labor necessary for the analysis of the calls and the other was deciphering the identity of the many unknown callers. The concept of night flight call monitoring clearly existed in the 1960s, but the method would need some technological developments before it was ready to advance further.

        A sign of the next technological advance was reported in 1987 by Volker Dierschke, studying the nocturnal flight call phenomenon on Helgoland Island in western Germany (Dierschke 1989). Peter Kaetsch, an engineer working on the project, developed a voice-activated nocturnal flight call recording system. The tape recorder was activated when it received a signal of specified loudness in the 3-8 kHz range. This conserved audio tape and also saved analysis time in allowing one to bypass the long sections of night when no calling occurred.

        Bill Evans became interested in studying nocturnal flight calls in 1985 and used commercial hi-fi video cassette recorders to record 8-10 hours of continuous sound from the sky per night. After years of laborious listening to tapes, Evans teamed with the Cornell Lab of Ornithology's newly formed Bioacoustics Research Program (BRP) in 1994 to work on the development of automatic nocturnal flight call detectors. Under the direction of Dr. Chris Clark, BRP was pioneering the development of computer-based (digital) acoustic analysis software. BRP programmer Harold Mills wrote the first functional nocturnal flight call detector in 1994 (Mills 1995). The software allowed automatic detection of the short, high-pitched flight calls of warblers and sparrows. This development allowed the automatic extraction of such flight calls from tapes or in real-time directly from an active microphone. BRP's spectrographic analysis software called Canary, written by Steve Mitchell, was then used to study and help discriminate similar calls. Much of the early work on in discriminating nocturnal flight calls in this CD-ROM was facilitated using Canary.

        In 1998, Evans founded the nonprofit organization called Old Bird, dedicated to facilitating nocturnal flight call monitoring. Old Bird contracted former BRP programmer Steve Mitchell to develop advanced software. New detectors were written that enabled operation on PC computers and allowed flexible settings for the frequency range where calls were to be detected. This included the first detector for thrush calls. In addition, software was developed for automatic recognition of one of the most distinctive avian night flight calls in eastern North America, the Dickcissel. Using Dickcissel call detection software, the first transect of automated night flight call recording stations was established in spring 2000 at a number of high schools in south Texas (see www.oldbird.org/dicks.html and Larkin et al. 2002). 

        The 21st century will likely see further advances in nocturnal flight call monitoring, and we encourage the reader to join the exploration by using the resources available on the Old Bird website to establish a monitoring station at their home or school.