ORIGINAL ARTICLE Magnetic compass of migratory Savannah sparrows is calibrated by skylight polarization at sunrise and sunset Rachel Muheim Æ Susanne A ˚ kesson Æ John B. Phillips Received: 31 December 2006 / Revised: 22 June 2007 / Accepted: 25 June 2007 Ó Dt. Ornithologen-Gesellschaft e.V. 2007 Abstract Migratory birds use compass systems derived from the geomagnetic field, the stars, the sun and polarized light patterns. We tested whether birds use a single underlying reference system for calibration of these com- passes and, specifically, whether sunset and sunrise polar- ized light cues from the region of the sky near the horizon are used to calibrate the magnetic compass. We carried out orientation experiments with Savannah sparrows, Passer- culus sandwichensis, in Alaska during autumn migration 2005, and compared the magnetic orientations of individual birds before and after exposure to conflicting information between magnetic and celestial cues. Birds exposed to an artificially shifted polarization pattern (±90° shift relative to the natural condition) for 1 h at local sunrise or sunset recalibrated their magnetic compass, but only when given access to the artificial polarization pattern near the horizon. Birds exposed to a 90° clockwise-shifted magnetic field for 1 h at solar noon did not recalibrate their magnetic com- pass. These results indicate that migratory birds calibrate their magnetic compass using the skylight polarization pattern vertically intersecting the horizon at sunrise and sunset. In conjunction with earlier work showing that sun and star compass calibrations are secondarily derived from magnetic and polarized light cues, our findings suggest that polarized light cues near the horizon at sunrise and sunset provide the primary calibration reference for the compass systems of migratory songbirds. Keywords Orientation Á Cue calibration Á Magnetic compass Á Skylight polarization Introduction Migratory songbirds use multiple compasses for orientation during migration, including magnetic, star, solar and polarized light compasses (e.g., Emlen 1970; Able 1982; Moore 1987; Schmidt-Koenig 1990; Munro and Wiltschko 1995; Wiltschko and Wiltschko 1995). Depending on which compass cues birds rely on during migration, they are led along different migration routes (Alerstam and Gudmundsson 1999; Alerstam 2001; Alerstam et al. 2001; Bingman et al. 2003; Muheim et al. 2003;A ˚ kesson and Hedenstro ¨m 2007). The relationships between the different compass routes change along the migratory journey due to changes in declination, resulting from differences in the relative positions of the magnetic and geographic poles. Since cue availability changes with weather conditions, season, time of day, and latitude, birds must calibrate the different compasses with respect to a common reference both before and during migration to avoid navigational errors. The common reference can be one of the compass systems that has primacy over the others, or an independent source of directional information. Despite three decades of intensive research, the reference system birds use, how they integrate information from the different compass systems, and how often the different compasses are cali- brated against each other are all still poorly understood. Communicated by H. Mouritsen. R. Muheim (&) Á J. B. Phillips Department of Biological Sciences, Virginia Polytechnic Institute and State University, Derring Hall, Blacksburg, VA 24061, USA e-mail: muheimr@vt.edu R. Muheim Á S. A ˚ kesson Department of Animal Ecology, Lund University, Ecology Building, 223 62 Lund, Sweden 123 J Ornithol DOI 10.1007/s10336-007-0187-4