Migrating birds use information extracted from the Earth’s magnetic field to target the same breeding grounds year after year, with the field’s inclination angle, in particular, acting as a “stop sign” telling them they have reached their destination. This conclusion, based on a new analysis of data obtained by attaching metal rings to the legs of birds and then tracking their movements, adds to a growing body of knowledge about field-based navigation in migratory animals.
Although there is considerable evidence that some species of birds use the Earth’s magnetic field to navigate, the precise mechanism is still not fully understood. Some theories invoke an inherited “clock and compass” vector system to explain how birds can return to their breeding grounds with extraordinary precision, but the question of how they know when and where to stop was unresolved.
In an attempt to answer it, Joseph Wynn and Tim Guilford of the University of Oxford in the UK and colleagues at the University of Oldenburg, Germany turned to a long-term experiment on Eurasian reed warblers (Acrocephalus scirpaceus). These migratory songbirds fly across the Sahara each year to spend the summer in Europe, and between the years 1940 and 2018 more than 17,000 of them were “ringed” to allow researchers to monitor their movements over many years and large geographical areas.
A single magnetic inclination angle
The team’s analyses suggest that the warblers register, or learn, a single magnetic inclination angle – effectively the degree of “dip” between the Earth’s magnetic field and its surface – before setting off on their journeys. Later, they use this angle as a unique coordinate telling them they have reached their breeding site. While different locations across the globe have the same inclination angle, the team say that the warblers solve this problem by stopping at the first place where they encounter the right inclination, according to their inherited vector system.
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This explanation may not be all there is to it, however. Because the Earth’s magnetic field in a given location changes slightly from one year to the next, the magnetic parameter values that are characteristic of an individual bird’s breeding site will exist in a somewhat different location on its return trip. In a paper in Science describing the work, the researchers acknowledge that these magnetic variations need to be taken into account. “Nonetheless, we believe that our findings provide evidence for an unconventional mechanism of long-distance navigation, both within birds and migratory animals more generally,” they conclude.
