Buzz
For centuries, people in the northern parts of the world have been puzzled by where birds go in winter and how they find their way there. Aristotle proposed that birds transformed into different species with the changing seasons; that redstarts turned into robins and garden warblers into blackcaps as the cold weather set in. As late as the 1800s, some naturalists believed that birds hibernated during the winter.
The truth is almost as wild as the idea of birds changing species with the seasons. Tens of thousands of bird species (around 40 percent of the planet’s birds) travel between summer nesting spots and winter retreats, often crossing continents and oceans to get there. A prime example is the arctic tern, which covers 44,000 miles annually, traveling from Greenland to Antarctica. Yet migration isn’t limited to just flying birds. Emperor penguins trek over 70 miles of ice each year to move from the sea to their breeding grounds.
There are various theories about how birds find their way on their seasonal journeys, and the full mechanics of their navigational systems remain somewhat of a mystery. For many birds, however, migration is an innate behavior, a journey their bodies instinctively prepare for when the time arrives.
As the days shorten in the fall, songbirds’ brains react to the diminishing sunlight, triggering hormonal shifts that prompt the birds to molt, increase their food intake, and crave the open sky. For example, bobolinks, a type of songbird, boost their food consumption by nearly 40 percent to prepare for their tough migration across the Caribbean Sea, ultimately swelling to 150 percent of their summer weight. Along with this need to feast comes a restlessness that drives them to take flight after sundown and keep flying through the night. This phenomenon is called zugunruhe in German. Even birds in captivity, who have no need or ability to migrate, experience this restlessness.
“As songbirds take flight just after sunset, the caged ones begin fluttering against their enclosures too,” explains ornithologist Miyoko Chu in her book Songbird Journeys (an essential resource for this article). “Their restlessness persists each night, only ceasing when the wild birds reach their winter homes.”
The direction of their flight is also ingrained in their instincts. A famous study from 1978 revealed that garden warblers raised in captivity flew in the same cardinal direction as their migrating counterparts, even though the captive birds had no visual access to the sky. Some migratory birds can detect magnetic fields and use them for navigation, though the precise mechanics are still not fully understood. For instance, bobolinks have magnetite in their nasal tissues, and studies of their brains show that vision-related neurons are activated when magnetic fields shift. In 2007, researchers at the University of Oldenburg discovered that garden warblers also appear to perceive magnetic fields, which may explain how the warblers in the 1978 study were able to determine the correct migration direction despite not seeing the sky.
The migratory direction is partly encoded in their genetics, leading to occasionally inefficient routes. In 2008, researchers Peter Berthold and Andreas J. Helbig crossbred birds with different migratory paths and found that their offspring struggled to determine the correct direction when migrating. These young birds attempted to fly a route that was halfway between their parents' routes, following conflicting instincts.
The migration route itself is not predetermined, however. Birds gradually learn how to navigate to their seasonal grounds, and younger birds may get lost along the way. In addition to using the Earth’s magnetic field, some birds also rely on the Sun and stars for guidance. Songbirds can detect polarized light patterns, which help them find their way. According to a 2013 study published in Biogeosciences, pigeons may also navigate using their sense of smell, memorizing specific scents in the air. If they become disoriented, they can retrace their flight by flying towards familiar odors, in reverse order from when they initially departed.
Some species struggle more with migration than others. For example, whooping cranes learn their migration routes from older, more experienced birds. A Canadian charity spent 15 years trying to teach captive whooping cranes how to migrate from Wisconsin to Florida for the winter. The young cranes would follow ultralight planes because, without guidance, they wouldn't know to leave the Midwest for warmer areas, nor where to go. Unfortunately, the program was recently shut down after the federal government withdrew its funding.
Environmental factors can greatly affect migration patterns. A study of thrushes by the Biological Station Rybachy in Russia found that while they are capable of flying through lightning storms, they will only take off if the weather conditions at sunset aren’t too harsh. If the temperature dips below 69°F or the winds exceed 6 mph, they will hold off and rest for the night. Additionally, if they haven’t gained enough weight for the journey, they’ll stop to feed and build up their fat reserves.
Since there are limited optimal migration paths, many bird species in the Western Hemisphere have adapted to travel along the same routes. These paths converge at various points between North and South America, where wind patterns and other environmental factors provide them with an advantage in crossing dangerous stretches of ocean.
Many birds return to the same territories year after year, often returning to the very area where they were hatched as chicks. Research shows that up to 60 percent of migratory songbirds return to the same place annually. So, if you spot a warbler, be sure to say "hi" – it’s likely to come back next year!
