Bird migrations have stumped the greatest minds for thousands of years. Aristotle thought that the robins living in Greece in the winter somehow turned into redstarts in the summer. In fact, robins migrate from Greece to Northern Europe around the time redstarts arrive from Africa.
Scientists have gotten a much better understanding of bird migration in recent centuries, but there’s a tremendous amount they have yet to learn. After tracking more than three dozen birds with sensors for thousands of miles, a team of researchers reported on Wednesday that their migration defied the expected course.
Instead of simply flying straight from their summer grounds in Denmark to their winter site in Africa, the birds stretched out their journey, stopping at several places along the way for weeks at a time.
“It’s more of a nomadic life,” said Kasper Thorup, a bird migration expert at the University of Copenhagen and co-author of the new study. “They hardly have a place to call home.”
The journey was exquisitely well timed to coincide with high levels of vegetation at each site, he and his colleagues found. These habits, honed by thousands of years of evolution, probably helped them enjoy a good diet of insects on their trip. This may be a common strategy among the world’s migratory birds, but Dr. Thorup and his colleagues warn that it may be threatened by climate change.
Previous generations of researchers could rely only on indirect clues to the travels of birds. Traditionally, ornithologists caught a bird at its summer breeding grounds, put an identifying band on its leg, and then waited for someone to spot it wherever it ended up for the winter. Such studies said little about where the birds went between points A and B.
Today’s migration researchers are finally filling in some of those gaps. Some are analyzing millions of crowdsourced bird sightings. Others are fitting birds with miniature tracking devices.
“We have all these resources coming online, and so we can replace speculation with observation,” said Frank A. La Sorte, a research associate at the Cornell Laboratory of Ornithology.
In 2002, the Cornell lab started a program called eBird with the Audubon Society. Amateurs fill out an online form each time they go birding, noting all the species they spot. Dr. La Sorte and his colleagues analyze the records, tracking a number of migratory birds that travel through North America each year.
The researchers have found that birds migrate in loops, rather than follow a straight line north and south. In summer, birds heading to eastern North America, for instance, catch tailwinds that help them get over the Gulf of Mexico.
The birds end up drifting to the west, but they still save energy despite the longer route. On the way back south, though, they take a more direct path across the Gulf.
In western North America, food plays a big part in the migration loop. Birds fly at night and then land to feed on insects during the day. In the spring, their routes take them to places where plants have put on a lot of growth. That’s probably also where the birds can find insect larvae to eat.
While eBird gives Dr. La Sorte information about millions of birds, it can show him only what birders are seeing. Other researchers are suiting up small squadrons of birds with tracking devices to follow them through their entire migrations.
For the new study, published in the journal Science Advances, Dr. Thorup and his colleagues put lightweight devices on common cuckoos. As the birds migrated between Denmark and Central Africa, they sent signals to satellites showing their location.
The researchers also tracked two smaller migratory species traveling from Denmark to sub-Saharan Africa: thrush nightingales and red-backed shrikes. These birds are too small to carry the weight of satellite transmitters, so Dr. Thorup and his colleagues fitted them with even tinier devices called geo-locators.
These sensors record only sunlight levels throughout each day. When the birds returned to Denmark, the scientists recovered the geo-locators and used each day’s data to plot the birds’ routes.
Studying 38 birds in total, the scientists found the animals didn’t move directly from their summer grounds to their winter grounds. Instead, the birds would fly for a few days, stop somewhere for a few weeks, and then move on again.
Red-backed shrikes, for example, leave Denmark and reach southeast Europe in August. In October, they go to East Africa. By December, they’re in southern Africa. And in April, they’re back in East Africa again.
Throughout their journey, the scientists found, the three species timed their flights so that they reached feeding grounds once they were abundant with vegetation. Dr. Thorup suspects that the birds are able to make meals of local insect larvae feeding on the plants.
The new data show that even though the birds ended up in the same places in Africa, they sometimes followed different routes. Dr. La Sorte and his colleagues have found similar flexibility in North American birds, which often adjust their route and speed when over the Gulf of Mexico or the Atlantic Ocean, perhaps to cope with changing weather conditions.
It’s possible that birds somehow combine short-range flexibility with a navigation system, hard-wired into the brain, that guides them to the places where they can find the most food to eat.
“You don’t cross the Sahara without knowing it’s good on the other side,” said Dr. Thorup.
That strategy works well when birds can be sure to find food at the same place at the same time each year. But climate change is altering the calculus.
In Northern Europe and North America, for example, plants are greening up earlier in the spring. In Africa, rainfall patterns are shifting, changing the times at which plants put on new leaves and fruit.
Dr. Thorup and his colleagues compared the migration routes of the birds to computer projections of how ecosystems will change in response to global warming over the next few decades.
They concluded that climate change may make migrations much harder on the birds.
In the summer, for instance, they will need to fly farther north to find food. “And when they arrive in Africa, there are no obvious places for them to go,” said Dr. Thorup.
Of course, birds have not migrated along the same paths for millions of years. Ice age glaciers gradually retreated from places like Denmark only about 11,000 years ago, allowing birds to colonize them for summer breeding.
But this change in bird migration was probably the result of natural selection acting on hundreds of generations of birds. Human-driven climate change is moving much more quickly.
“We’re worried that they won’t have the time to adapt,” Dr. Thorup said.
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