Cochran, W. W., Mouritsen, H., & Wikelski, M. (2004). Migrating songbirds recalibrate their magnetic compass daily from twilight cues. Science, 304(5669), 405–408.
Abstract: Night migratory songbirds can use stars, sun, geomagnetic field, and polarized light for orientation when tested in captivity. We studied the interaction of magnetic, stellar, and twilight orientation cues in free-flying songbirds. We exposed Catharus thrushes to eastward-turned magnetic fields during the twilight period before takeoff and then followed them for up to 1100 kilometers. Instead of heading north, experimental birds flew westward. On subsequent nights, the same individuals migrated northward again. We suggest that birds orient with a magnetic compass calibrated daily from twilight cues. This could explain how birds cross the magnetic equator and deal with declination.
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Froy, O., Gotter, A. L., Casselman, A. L., & Reppert, S. M. (2003). Illuminating the circadian clock in monarch butterfly migration. Science, 300(5623), 1303–1305.
Abstract: Migratory monarch butterflies use a time-compensated Sun compass to navigate to their overwintering grounds in Mexico. Here, we report that constant light, which disrupts circadian clock function at both the behavioral and molecular levels in monarchs, also disrupts the time-compensated component of flight navigation. We further show that ultraviolet light is important for flight navigation but is not required for photic entrainment of circadian rhythms. Tracing these distinct light-input pathways into the brain should aid our understanding of the clock-compass mechanisms necessary for successful migration.
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Muheim, R., Phillips, J. B., & Akesson, S. (2006). Polarized light cues underlie compass calibration in migratory songbirds. Science, 313(5788), 837–839.
Abstract: Migratory songbirds use the geomagnetic field, stars, the Sun, and polarized light patterns to determine their migratory direction. To prevent navigational errors, it is necessary to calibrate all of these compass systems to a common reference. We show that migratory Savannah sparrows use polarized light cues from the region of sky near the horizon to recalibrate the magnetic compass at both sunrise and sunset. We suggest that skylight polarization patterns are used to derive an absolute (i.e., geographic) directional system that provides the primary calibration reference for all of the compasses of migratory songbirds.
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