Nocturnal Migrant Flight Call Research
Twice a year millions of birds migrate across the Americas to and from ancestral breeding and wintering grounds. For most species, the migrations occur under cover of night, and for thousands of years this passage has been mostly clandestine to all creatures but those who listened to the birds' nocturnal flight calls.
Many species of birds vocalize during their night migrations. On a good migration night in eastern North America, listeners on the ground may hear thousands of calls. We hypothesize that the calling helps birds form and maintain in-flight associations. In the process of calling, they organize their spacing and avoid collisions. Historical research on avian night flight calls has given us a glimpse of the great potential for gaining information on night-migrating birds by identifying and counting their nocturnal flight calls.
The nocturnal flight call research program at the Lab was initiated in Fall 1994, based on ground-breaking research by Bill Evans, then a Lab associate.
In 2000 and 2001 the Cornell Lab of Ornithology embarked on a project for automatically monitoring migrating birds along the Eastern seaboard in conjunction with the Clemson University Radar Ornithology Laboratory, National Audubon Society, and the Academy of Natural Sciences. The BirdCast project was an exciting and novel collaborative effort that used NEXRAD radar imagery to depict bird movements throughout peak migration periods. As part of the BirdCast project, the Bioacoustics Research Program, in conjunction with a number of volunteers, operated a network of five acoustical migration monitoring stations in Pennsylvania, Maryland, and New Jersey in 2000. Each station, which consisted of a microphone (Figure 3, below) and a computer running automatic acoustical transient detection software, listened to the night sky for short, high-pitched sounds like those produced by migrating warblers and sparrows. Whenever such a sound was detected, it was automatically saved to a disk file for further analysis. BRP staff uploaded these sounds over the internet and identified the flight-calls to species or species group.
This was an exciting technological step toward automatic monitoring of nocturnal flight-calls on a continent-wide scale. Just as weather data across our continent is transformed into the user friendly maps we see on TV, we could watch bird migration TV!
Figures 1 and 2: Recordings made at night during spring migration yielded these examples of nocturnal flight calls of Black-and-white Warblers (Figure 1.) and American Redstarts (Figure 2.), depicted here as spectrograms. The durations of these calls are about 1/10th of a second.
Figure 3: Eastern Bluebirds find this developmental BirdCast microphone installation (named "SkyEar") useful for purposes other than those intended.
- Farnsworth, Andrew, 2005, "Flight Calls and their Value for Future Ornithological Studies and Conservation Research," The Auk, pp. 733–746.
- "Tuning in to Invisible Migrations," BirdScope, Winter 2007.
- Evans, W. R. and K. V. Rosenberg. 2000, "Acoustic Monitoring of Night-Migrating Birds: A Progress Report," IN: Strategies for Bird Conservation: the Partners in Flight Planning Process. Proceedings of the 3rd Partmers in Flight Workshop; Cape May, NJ. October 1-5, 1995.