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 due to highly variable and apparently contradictory results obtained in various experimental studies. How birds decide whether or not 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 warblers 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° clock/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 of 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.

Previously it has been shown that migratory birds were oriented in the appropriate migratory direction under UV, blue and green monochromatic lights (short-wavelength) and were unable to use their magnetic compass in total darkness and under yellow and red lights (long-wavelength). Currently, it is generally assumed that the magnetic compass of birds works correctly only under short-wavelength light. However, at the same time, there is an assumption 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), in different monochromatic lights during autumn migration. The birds were tested in the natural magnetic field (NMF) and 120° CCW shifted magnetic field (CMF) under green and yellow light (intensity 1 mW m-2). All tests were performed in a specially constructed wooden laboratory equipped with magnetic coils to manipulate the magnetic field. We showed (1) pied flycatchers were completely disoriented under green light both in the NMF and CMF and (2) for the first time they showed the migratory direction in NMF and appropriate response to CMF under yellow light. Our data suggest that the avian magnetic compass might be based on two different mechanisms: a high-sensitive short-wavelength mechanism and a low-sensitive mechanism in the long-wavelength spectrum.