migratory birds don't have a compass because of the Earth's magnetic field.
The biological gadgets known as magnetoreception allow birds to know which direction to head in on their first outbound migration but also to know how to return to their original site with extreme accuracy.
Scientists investigated if birds use Earth's magnetic field to locate their breeding sites more accurately.
The dip angle between Earth's magnetic field and Earth's surface could be a clue.
If an extreme weather event had sent birds off course, earlier research suggested similar ideas.
The data from 17,799 ringing recoveries was used to investigate if and how the reed warbler used magnetic information to return to its home.
If these birds actually use Earth's magnetic field to locate their original breeding site, then the researchers believe that yearly variations in Earth's magnetic field should be reflected in gradual changes of the bird's nest areas.
The authors explain that because Earth's magnetic field varies slightly year by year, the magnetic parameters of an individual's breeding site will be different the following year.
If birds used magnetic parameters to determine the location of their natal or breeding site, we would expect that the location of specific magnetic parameters would change over time.
Findings from the study show that magnetic inclination is the primary magnetic cue for the birds when relocating their breeding site, with specific inclination parameters signaling as a sort of stop sign.
The inclination angle is suggested to be learned by the birds before leaving their breeding sites.
The authors theorize that this is consistent with inclination acting as a uni-coordinate stop sign and that birds could recall their location using only one coordinate.
Magnetic inclination has the most stable year-on-year variation compared to other potential magnetic cues, so it makes sense to use it as the primary cue for relocating their breeding site. It gives migratory birds a sign that they have reached a desirable location.
The proposed mechanism is relatively robust because other magnetic gradient positions move further with secular variation. The position of the site would move on average 18 km between years, as estimated using intensity and declination, 20.4 km, and as estimated using intensity and inclination, 98.2 km.
The location of the breeding site uses inclination as a stop sign and moves only 1.22 km between years. We suggest that by remembering breeding location relative to the most stable cue and referencing it alongside a compass bearing, the proposed strategy reduces the impact of secular variation.
Scientists found that birds recovered for use in the study closer to the site predicted by the inclination stop sign model than they were to their natal or breeding site, suggesting the birds may even prioritize the bio coordinate clues of magnetic inclination over even their breeding site.
It appears that migratory birds are able to navigate and find the crucial environments needed for their continued survival by using a number of biological mechanisms related to the parameters determined by Earth's magnetic field.
The journal Science published the research.
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