Migratory birds are known for their ability to traverse thousands of kilometres to reach their breeding or wintering grounds. Research by Bangor University found that these birds, in this case, Eurasian reed warblers (Acrocephalus scirpaceus) are using only the Earth's magnetic inclination and declination to determine their position and direction. This challenges the long-held belief that all components of the Earth's magnetic field, especially total intensity, are essential for accurate navigation.

Scientists have long believed that these birds use a 'map-and-compass' system: they first determine their location using a 'map' and then use a 'compass' to orient themselves in the correct direction. However, the exact nature of this 'map' has been the subject of ongoing debate.

In a carefully designed experiment, warblers were exposed to artificially altered magnetic inclination and declination values, simulating a displacement to a different geographic location while keeping the total magnetic intensity unchanged.

Despite this 'virtual displacement', the birds adjusted their migratory routes as if they were in the new location, demonstrating compensatory behaviour. This response suggests that birds can extract both positional and directional information from magnetic cues, even when other components of the Earth's magnetic field, such as total intensity, remain unchanged.

The research provided strong evidence that migratory birds rely on inclination and declination to determine their location, even when these cues conflict with other magnetic field components.

"What's interesting is that these findings reveal that the birds don't necessarily need all components of the Earth's magnetic field to determine their position," said Professor Richard Holland, who specialises in animal behaviour and led the study. "They can rely solely on inclination and declination, which are also used in compass orientation, to extract their location."

The study challenges previous assumptions that all components of the Earth's magnetic field, particularly total intensity, are necessary for accurate navigation. "It remains to be seen whether birds use the total intensity of the Earth's magnetic field for navigation in other contexts, but what we've shown is that these two components -- magnetic inclination and declination -- are enough to provide positional information," explained Richard.

This discovery advances the understanding of avian navigation and supports the theory that birds possess a complex and flexible internal navigation system. This mechanism allows them to adjust for changes in their environment, even when encountering conditions they've never experienced before.

The findings open new avenues for research into animal navigation and may hold implications for broader biological studies, including how animals interact with and interpret their environment.