ИСТИНА

Migrating songbirds are able to navigate across the surface of the Earth. Experienced migrants use a map based on long-range environmental cues because they can correct for displacements to unknown areas. However, it has remained a mystery how migrants determine their longitude (i.e. east-west) position. Latitude (i.e. north-south position) can be inferred from magnetic inclination, total magnetic intensity and, theoretically, also from the angle of the celestial point of rotation above the horizon. Detection of magnetic declination, the angular difference between magnetic and geographic north, would require very accurate celestial and magnetic compasses, but could provide the missing east-west cue making the bird’s map usable on a continental scale. Here we used virtual magnetic displacements to test whether adult and first-autumn Eurasian reed warblers (Acrocephalus scirpaceus) and first-autumn European robins (Erithacus rubecula) can detect magnetic declination. Our results suggest that experienced migrants can detect changes in magnetic declination and respond in a manner consistent with compensation for the virtual magnetic displacement. Naive migrants either did not respond to the changed magnetic declination at all or increased scatter in their orientation. Thus, a migrating songbird can detect magnetic declination, and it provides the missing east-west map coordinate that enables experienced migrants to navigate in virtually all areas of the globe regularly frequented by migratory songbirds.