Surprisingly, most bird species migrate at night. This makes understanding and quantifying migration more difficult than pulling out your binoculars in spring and fall. Several techniques have been developed to help understand night migration. Each technique has it’s strengths and weaknesses.
Avian acoustic monitoring is a little-used technique in surveys for wind energy. In some situations, it can be very helpful. Most birds can be identified to species using their night-migration calls. Acoustic monitoring can therefore be used to determine species composition during the night. It can also be useful in identifying night-migrating endangered species in an area. For example, some species may migrate through farmlands during the evening, but be completely absent during “normal” daytime survey hours. It is difficult to quantify risk for these species using standard point-count surveys.
Avian acoustic monitors are typically placed at ground level or slightly above (which helps reduce noise from insects). Microphones can be directional or not, but in the “classic” set up (Evans 2003), a non-directional microphone is set up within a flower pot, resulting in a semi-directional effect. Modern microphones can pick up most typical bird calls at around 300ft (this varies due to microphone, temperature, moisture, insect noise, and bird call volume). Turbine typically stand about 400 to 500ft, with the blade reaching to within 150ft of the ground, so the acoustic set-ups will pick up most but not all birds passing through the rotor swept area. Like most detectors, the area sampled is relatively conic, or cone-shaped, so the sample space is larger higher up.
As with many other passive detectors (such as acoustic bat monitors) neither the area sampled or the number of birds passing overhead are simple measures. Birds with louder calls can be heard from further away, as can birds with a call pitch that more closely matches the optimum performance range of the microphone.
The number of bird calls is not a directly related to the number of birds. Some birds call a lot, while others tend to remain relatively silent. For example, terns are heard very infrequently during migration, while thrushes are the predominant group observed on the eastern seasboard during much of the fall migration. However, you can get an idea of the relative use by comparing the activity level of the same bird species in different areas.
Finally, while many birds can be identified to species using call notes, not all species can be identified yet. Identifying birds by sight is a difficult enough skill to requires an experienced birder. Some birders can identify birds by song. Nighttime call notes, however, are of very short duration and can be very difficult to identify (anyone with experience trying to identify sparrows by “chip” notes can attest to this!). Fortunately, Bill Evans and several people at Cornell Acoustic labs are making call data available to scientists.
At Merlin Ecological, we are currently conducting avian acoustic surveys to better understand neotropical warbler migration. We hope to present our findings later in 2011.