What Are Night Flight Calls?
Every spring and every fall, millions of birds cross our skies as they migrate between their wintering grounds and breeding grounds. Many of these migrations are made under the cover of night, and go unnoticed by the people below. Although we cannot see them pass, we can hear them as they make flight calls.
Flight calls are different from a bird's normal song and call notes and almost exclusively used by birds during night flights. They are thought to be used in two ways. One is that they help keep a flock of migrating birds of the same species together. A bird calls and listens for other calls to make sure it is staying with its flock and not heading in the wrong direction. Flight calls also act as a form of air traffic control. Birds listen for other birds to prevent in-air collisions. The frequency of flight calls given by birds increases as visibility decreases. Birds caught in fog or clouds will call much more then birds flying in clear skies under a full moon.
Flight calls for most birds are extremely short - only a half of a second in length or shorter. Many calls are often given at such a high pitch that it can be difficult for the human ear to detect them, particularly if there is a lot of ambient noise around. Shorebirds and some waterfowl may have louder, easier-to-detect calls - many people are familiar with the sounds of killdeer, geese, and swans passing over during the night.
Simple microphones can be made to listen in on the quieter flight calls. When these microphones are hooked up to a computer, a record can be made of the birds that are migrating during the night. Scientists are using computer analysis of flight calls to get a better understanding of when birds migrate and what routes they use. Monitoring flight calls can also quickly alert scientists to changes in a species' population. The study of flight calls is still a relatively new area of research, and more applications for this reseach may be discovered.