The sounds of migration
How we do it
THE SOUNDS OF MIGRATION
Although we can determine when birds are responsible for the echoes NEXRAD detects, we cannot discern which species they are by using radar images. One way to identify species is to listen to the sounds of migration. That's right! Many songbird species call during their nightly migratory flights.
Typically, songbirds that migrate at night do so individually, or in very loose aggregations, and not in flocks. We think nocturnal migrants use calls to stay in touch with other species members during the actual flight, possibly as a way of staying on course. Calling also appears to increase toward the end of a night's migration. Migrants may use calling to locate species members that have already landed.
Every species has a unique call. A visual representation of the call, or spectrogram, can be used to make species identifications. By recording the flight calls of migrating birds, and studying their associated spectrograms, we can characterize the species composition of migration events we detect on NEXRAD.
Currently, New Jersey Audubon Society collects radar data from the DOVER AFB, DE (DOX, 38.83o N, 75.44o W) and the Philadelphia, PA (DIX, located at Fort Dix, NJ, 39.95o N, 74.41o W) NEXRAD stations (Figure A). These two stations give us extensive radar coverage of southern, central, and north/central NJ.
We also operate six "flight call" detection stations in southern New Jersey. Five of these stations are located at destination points along the NJ Coastal Heritage Trail Route.
Two flight calls are shown below of Indigo Bunting and Baltimore Oriole. (Click on the sonogram to hear the calls.)
HOW WE DO IT
We use a device called a pressure zone microphone (PZM) to record nocturnal flight calls. In a PZM, the microphone element is placed very close to a rigid boundary. The incoming sound wave is received by the microphone element at almost the same time as it receives the sound wave being reflected from the boundary (RIGHT).
The result is a doubling of sound pressure near the microphone element. The size of the rigid boundary (in our case, a plastic dinner plate) determines the range of frequencies where the sound pressure is doubled.
In our PZMs, the microphone element is mounted to the top of the dinner plate and soldered into a simple circuit mounted to the bottom of the plate. The microphone and circuit run on a 9V battery.
The entire microphone/plate assembly is sealed with plastic wrap to prevent moisture from damaging the circuitry. Our PZMs are housed in large plastic flower pots anchored to the roof of a building at a host location.