Capturing reed warblers from the wild, researchers were able to prove the birds’ ability.
(CN) — Setting out on their first-ever journey, migratory birds have a compass but not a map.
Born with an innate directional ability, first-year birds like the Eurasian reed warbler can make their inaugural migration, spanning thousands of miles, heading south for winter feeding and north in the summer to breed.
During that first trip, the birds are doing something else key to their survival in future journeys: They are creating an internal map based on Earth’s magnetic field.
“The theory behind it would be that they’d have some sort of gradient map, that essentially was doing something similar to what our Cartesian coordinates do,” said Richard Holland, professor of animal behavior at Bangor University in Wales.
“It can’t be explained by the kind of navigation that we use,” like learning landmarks and directions in space, Holland explained, because these birds are able to navigate to places they’ve never been before — locations far off the grid map the birds create.
Holland co-authored a new study showing the range of that avian ability for the first time in Eurasian reed warblers.
The study, published Friday in the journal Current Biology, builds on previous work to show that birds can course-correct by extrapolating their geomagnetic knowledge, even if they wind up far from their usual path.
“They seem to have this remarkable precision to return back to the same place,” Holland said of the birds during a video interview. “You can take them far off their normal migratory route, and so long as they’ve made that journey once before, they can get back.”
Earth’s magnetic forces create a geomagnetic “signature” based on their location on the planet. The forces vary in intensity along a north-south axis. They also change according to the angle formed by lines of the magnetic field meeting the Earth, and the angle created by the difference between directions to the geographic and magnetic North poles.
Sensing those multidimensional variations helps birds correct course if they deviate far from their migration pattern, increasing their odds of surviving the migratory journey.
In the now-published experiments, researchers captured Eurasian reed warblers that had already made a migration, and thus they knew the lay of the land. (The birds were released after experiments.)
They used what’s called virtual displacement: “We don’t move the bird, the way the original experiments did, we move the magnetic field,” Holland said.
Placing the birds in a funnel-shaped chamber, the researchers used electricity and copper wires to manipulate the magnetic forces surrounding the birds. Since everywhere on Earth has its own geomagnetic signature, “we just changed the magnetic field surrounding the birds to a different place,” Holland said.
Importantly, and as other experiments have shown, the birds have a tendency to try to escape in the direction that they would be migrating in the wild. Birds that Holland studies at a site in Austria, for example, will scratch in the direction of South America, to which they would ordinarily migrate during the winter.
Noting the direction in which the warblers tried to escape their chambers, the researchers were able to determine that even when “placed” in a totally unfamiliar location, the birds knew the path to their home journey.
“They must be able to essentially extend their range,” Holland said. Birds like warblers, in other words, have an ability to extrapolate beyond what they can learn about Earth’s magnets during migration.
“That’s what we’ve always thought they could do,” noted Holland. His team’s experiment, though, is the first one to prove it.