Migratory birds are able to choose the appropriate migratory direction under dim yellow narrowband light.

Currently, it is generally assumed that migratory birds are oriented in the appropriate migratory direction under UV, blue and green light (short-wavelength) and are unable to use their magnetic compass in total darkness and under yellow and red light (long-wavelength). However, it has also been suggested that the magnetic compass has two sensitivity peaks: in the short and long wavelengths, but with different intensities. In this project, we aimed to study the orientation of long-distance migrants, pied flycatchers (Ficedula hypoleuca), under different narrowband light conditions during autumn and spring migrations. The birds were tested in the natural magnetic field (NMF) and a changed magnetic field (CMF) rotated counterclockwise by 120° under dim green (autumn) and yellow (spring and autumn) light, which are on the 'threshold' between the short-wavelength and long-wavelength light. We showed that pied flycatchers (i) were completely disoriented under green light both in the NMF and CMF but (ii) showed the migratory direction in the NMF and the appropriate response to CMF under yellow light. Our data contradict the results of previous experiments under narrowband green and yellow light and raise doubts about the existence of only short-wavelength magnetoreception. The parameters of natural light change dramatically in spectral composition and intensity after local sunset, and the avian magnetic compass should be adapted to function properly under such constantly changing light conditions.

Access to the sky near the horizon and stars does not play a crucial role in compass calibration of European songbird migrants.

Migratory birds use different global cues including celestial and magnetic information to determine and maintain their seasonally appropriate migratory direction. A hierarchy among different compass systems in songbird migrants is still a matter for discussion owing to highly variable and apparently contradictory results obtained in various experimental studies. How birds decide whether and how they should calibrate their compasses before departure remains unclear. A recent 'extended unified theory' suggested that access to both a view of the sky near the horizon and stars during the cue-conflict exposure might be crucial for the results of cue-conflict experiments. In this study, we performed cue-conflict experiments in three European songbird species with different migratory strategies (garden warbler, Sylvia borin; pied flycatcher, Ficedula hypoleuca; and European robin, Erithacus rubecula; juveniles and adults; spring and autumn migrations) using a uniform experimental protocol. We exposed birds to the natural celestial cues in a shifted (120 deg clockwise/counterclockwise) magnetic field from sunset to the end of the nautical twilight and tested them in orientation cages immediately after cue-conflict treatments. None of the species (apart from adult robins) showed any sign of calibration even if they had access to a view of the sky and local surroundings near the horizon and stars during cue-conflict treatments. Based on results of our experiments and data from previous contradictory studies, we suggest that no uniform theory can explain why birds calibrate or do not calibrate their compass systems. Each species (and possibly even different populations) may choose its calibration strategy differently.