Birds use the Earth’s magnetic field for “stop signs” when migrating

Thanks to a combination of visual detection of the earth’s magnetic field and an integrated compass that allows them to orient themselves according to the magnetic intensity, migrating birds do not have much difficulty in finding their way.

These biological gadgets, known as magnetoreception, allow birds to not only know which direction to head on their first outward migration, but also know how to return to their nesting sites with pinpoint accuracy, often a few meters from their original natal site.

To understand how birds know when to stop, scientists investigated whether birds could also use signals from the Earth’s magnetic field to more precisely locate their breeding sites.

The clues could be magnetic tilt – the angle of inclination between the Earth’s magnetic field and the Earth’s surface – or magnetic intensity, the overall strength of the Earth’s magnetic field.

Previous research has suggested similar ideas as a way for birds to return to their desired flight paths if an extreme weather event caused them to veer off course.

The new study used data from 17,799 banding (tagged bird) recoveries from 1940 to 2018 to determine if and how the reed warbler (Acrocephalus scirpaceus), a trans-Saharan migratory songbird, used magnetic information to return to its nesting site.

If these birds actually use cues from the Earth’s magnetic field to locate their original breeding site, the researchers believe that yearly variations in the Earth’s magnetic field should be reflected in the gradual changes in position of the bird’s nesting areas.

“Because the Earth’s magnetic field shifts slightly from year to year, the values ​​of magnetic parameters characteristic of an individual’s natal or breeding site will exist at a different location the following year,” the authors explain.

“Therefore, if birds used magnetic parameters to determine the location of their natal or breeding site, we would expect changes in position between years to reflect year-to-year changes in the location of specific magnetic parameters.”

The results of the study indicate that magnetic tilt is the primary magnetic cue for birds when moving from their breeding site, with specific tilt parameters signaling as a sort of “stop sign.”

The authors suggest that birds “learn” the angle of inclination before leaving their breeding grounds.

“We hypothesize that this is consistent with tilt acting as a unicoordinate stop sign: birds could remember their birth or breeding location using a single coordinate dimension, if used alongside a compass bearing linking wintering and breeding sites,” the authors state.

According to the authors, using magnetic tilt as the primary cue for moving their breeding site makes sense, as it exhibits the most stable year-to-year variation compared to other potential magnetic cues. It provides migrating birds with a more reliable sign that they have reached a desirable location.

“In addition, other positions derived from the magnetic gradient move further with secular variation, making the proposed mechanism relatively robust. The position of the natal site as estimated using inclination and declination as a map bicoordinate would move, on average, 18.5 km (11.5 miles) between years; as estimated using intensity and declination, 20.4 km; and as estimated using l intensity and inclination, 98.2 km,” the authors state.

“In contrast, the location of the breeding site shown using incline as a stop sign only moves 1.22 km between years. We suggest that by remembering the breeding site relative to the most stable index and by referencing it alongside a compass bearing, the proposed strategy minimizes the impact of secular variation.

In fact, the scientists found that the birds retrieved for use in the study were closer to the site predicted by the tilt stop sign model than they were to their natal or breeding site, suggesting that birds may even prioritize biocoordinate cues of magnetic tilt even at their breeding site.

Overall, it appears that by harnessing a number of biological mechanisms related to parameters determined by the Earth’s magnetic field, migratory birds are able to successfully navigate and find the crucial environments necessary for their continued survival.

The research was published in the journal Science.

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