Avian Migration: The Ultimate Red-Eye Flight
Birds that migrate at night enter a state of sleepless mania and gorge on foods by day, behaviors mediated by their biological clocks
Imagine yourself on board a red-eye flight from Los Angeles to New York City, an eight-hour journey that begins at bedtime and ends at breakfast. Your plan to sleep during the flight is thwarted by sporadic turbulence and an uncomfortable seat. When you arrive at John F. Kennedy Airport, you feel dehydrated and grumpy, but you head straight to work for an important meeting. Fast food, caffeine and deadlines fuel your day’s full schedule. That night, you order Chinese takeout and eat it mindlessly in front of your laptop. You want nothing more than a warm shower and a long rest. Unfortunately, it’s time to head back to the airport for another red-eye flight.
Although such a schedule is far from ideal, it’s manageable every once in a while. But imagine for a moment that this is your daily routine—working by day and flying by night, for weeks on end. Imagine also that there are no drinks or food on the plane. Oh, and you are powering the flight by riding a stationary bicycle.
Of course, this is absurd and impossible. Yet billions of birds perform an analogous routine twice a year as they migrate between summer breeding grounds and wintering grounds. Of the 700 or more bird species nesting in North America, more than 400 species migrate. Worldwide, migratory birds are declining faster than nonmigrants. Understanding the challenges that migratory species face is an important conservation issue.
Migration requires dramatic seasonal changes in behavior and physiology, and these changes must be timed appropriately for successful migration. In late summer after nestlings fledge, birds begin to molt, replacing their ratty old feathers with sleek new ones. They also begin to gorge themselves. The flurry of activity around this time of year reflects this frantic, single-minded pursuit of food. The birds’ hyperphagia, or excessive eating, is accompanied by great changes in body weight and composition. The birds get very fat—and then they are gone, en route to their wintering grounds on a journey of several weeks. They spend the winter in warmer climates, where resources are sufficient for survival. In late winter, they grow new feathers again; afteward, there’s another weeks-long period of hyperphagia. When the days get longer and the temperature is just right, they’re off again, migrating to summer breeding grounds. Upon arrival, males establish territories. Pairs form. Nests are built. Soon, eggs are incubating, then hatching, and parents devote almost all of their energy to feeding chicks. If time permits, parents may mate again and have another clutch. Then, the cycle repeats (see Figure 2 below).
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.
Video courtesy of Paul Bartell