Birds chase an eternal spring on long migrations

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 recently reported 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 of 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 Kasper and his colleagues warn that it may be threatened by climate change.

Tracking the route
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 analysing 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 of the Cornell Laboratory of Ornithology, USA. In 2002, the Cornell lab started a programme called eBird with the National Audubon Society, USA. Amateurs fill out an online form each time they go birding, noting all the species they spot. Frank and his colleagues analyse 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.

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, Kasper and 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 travelling 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 Kasper 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, 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.

The new data show that even though the birds ended up in the same places in Africa, they sometimes followed different routes. Frank 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.

Changing ecosystems
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.

Kasper 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 for food. “And when they arrive in Africa, there are no obvious places for them to go,” said Kasper. 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 colonise 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,” Kasper said.