Listening In On Night Flying Songbirds
When songbirds travel between their winter grounds in the South and their summer grounds in North America they usually fly at night. These migrations are often heroic undertakings involving thousands of birds in mixed flocks flying several thousand miles. Why do they fly at night? There are many possible reasons: they are safer from predators at night; they can use the daylight hours to find food and rest; nighttime air is usually less turbulent making it easier for them to fly. Since they fly at night, many migratory bird species use starlight to find their way over immense distances. This is why aviation safety lights on towers and buildings are a hazard for them: on cloudy nights safety lights on towers can attract them.
During their migration flights many of the birds emit short vocalizations that sound somewhat like cricket chirps. Since they are flying in large mixed flocks the flight calls probably keep groups together and keep individuals from flying into one another. These flight calls are distinctive to each species of bird, so if one were to record these calls it should be feasible to estimate how many birds of each species flies over in a night. William Evans has spent the last seventeen years developing this idea into a useful conservation tool. We spoke with him about his work to increase our understanding of these amazing animals.