Fly by Night
Bird migration at Eddystone Lighthouse, by Charles Samuel Keene
From American Scientist:
Migration likely brings to mind the familiar sight of geese flying overhead in their iconic V formation, honking stridently as they fly toward their faraway goal. But the migration of many birds is a rarely observed phenomenon. Most passerine birds, a group that includes songbirds and groups taxonomically related to them, migrate at night. Nocturnal migration has fascinated scientists and bird enthusiasts for a long time. What are the advantages for birds that migrate at night? How do they do it? When do they sleep? The answers to these questions are as yet incomplete. And often answers only beget more questions. Nevertheless, technological advances have facilitated a recent surge in migration research. A recurring theme of this work is that biological clocks are intimately involved in controlling nocturnal migration.
How do we know birds migrate at night? For a long time, people have observed that flocks of birds change location between evening and the following morning. Since around 1880, ornithologists have used lunar observation—watching birds fly past the moon—to document nocturnal flights. A tally of nocturnal flight calls was published in 1899, although this technique did not flourish until the 1950s, when advances in sound recording made it more practical. During the early days of radar technology in the 1940s, “phantom signals” were discovered to be migrating birds. Radar has since become a widely used tool for monitoring bird migrations. Many of these classic methods are still used, with some modern improvements. For example, with the aid of special microphones and automated sound detection software, ornithologists recently reported in the Wilson Journal of Ornithology that pine siskins (Spinus pinus) undergo an irregular, nomadic type of nocturnal migration. Nocturnal migration may be more widespread than previously thought.
Nocturnal migratory activity is also studied in the laboratory. In captivity, night-migrating birds display stereotypic migratory behaviors during the night known as Zugunruhe, meaning “migratory restlessness.” Birds exhibiting Zugunruhe flap their wings rapidly as if about to take off from the perch. The term was coined by German bird fanciers who caught and kept wild birds; they noticed that at night, during certain times of year, their birds’ migratory proclivities resulted in damage to their feathers. This wing-whirring behavior can be clearly distinguished from captive birds’ daytime behaviors, such as hopping or feeding. Zugunruhe occurs during the dark period only. Because Zugunruhe behavior is maintained in the laboratory, biologists have been able to study diverse subjects related to migration, including biological clocks, navigation, metabolism and sleep.