Magnetic orientation of marsh warblers (Acrocephalus palustris) and spotted flycatchers (Muscicapa striata) after simulated crossing of the magnetic equator.

Every year, billions of birds migrate to optimize their foraging, shelter and breeding. They use an inclination compass, which, unlike the technical compass, distinguishes between the directions towards the magnetic equator from the magnetic pole based on magnetic inclination angles, which range from ±90 deg at the poles to 0 deg at the equator. During autumn migration, some species cross the magnetic equator, where field lines are horizontal, i.e. the inclination angle is 0 deg. At this point, the avian magnetic compass becomes ambiguous, because the birds can no longer distinguish 'to the pole' from 'to the equator'. Experiments with bobolinks and garden warblers have shown that these birds adaptively change their orientation when exposed to a horizontal magnetic field. We tested this in marsh warblers and spotted flycatchers, but they showed no such response, suggesting they may use other cues. This indicates that different species may rely on varying stimuli, and the current experimental models may not be universally applicable.