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.

Determining the East-West position was a classical problem in human sea navigation until accurate clocks were manufactured and sailors were able to measure the difference between local time and a fixed reference to determine longitude. Experienced night-migratory songbirds can correct for East-West physical and virtual magnetic displacements to unknown locations. Migratory birds do not appear to possess a time-different clock sense; therefore, they must solve the longitude problem in a different way. We showed earlier that experienced adult (but not juvenile) Eurasian reed warblers (Acrocephalus scirpaceus) can use magnetic declination (the difference in direction between geographic and magnetic North) to solve this problem when they were virtually displaced from Rybachy on the eastern Baltic coast to Scotland. In this study, we aimed to test how general this effect was. Adult and juvenile European robins (Erithacus rubecula) and adult garden warblers (Sylvia borin) under the same experimental conditions did not respond to this virtual magnetic displacement, suggesting significant variation in how navigational maps are organised in different songbird migrants.

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.

Bats, like birds, are capable of long annual migrations; however, a very limited number of studies are devoted to the role of the Earth's magnetic field in bat navigation. We aimed to perform a series of experiments on Nathusius's pipistrelle (Pipistrellus nathusii) to ensure that they are able to use the geomagnetic field for orientation. Bats were tested under two different conditions: in the geomagnetic field and the field, rotated 120 deg CW. To determine the takeoff direction and analyse behaviour in different magnetic conditions, we used the modified circular release box (CRBox) and a mini camera with IR LEDs. Helmholtz magnetic coils were used to manipulate the magnetic field. Bats were captured during migration through the Curonian spit (Kaliningrad region, Russia). Totally 53 bats were tested during August and September 2021-2022. During the second year, we recorded post-release bat's behaviour using a thermal camera. Although results from 2021 are ambiguous, data obtained in 2022 suggests that under given conditions bats, unlike birds, could prefer local audible landmarks and wind direction prior to global cues. However, the recordings of released bats clearly show that they have some specific directional preferences, which correspond to their migratory orientation.