White-throated sparrows calibrate their magnetic compass by polarized light cues during both autumn and spring migration

Abstract

The interaction and hierarchy of celestial and magnetic compass cues used by migratory songbirds for orientation has long been the topic of an intense debate. We have previously shown that migratory Savannah sparrows, Passerculus sandwichensis, use polarized light cues near the horizon at sunrise and sunset to recalibrate their magnetic compass. Birds exposed to a +/-90 deg. shifted artificial polarization pattern at sunrise or sunset recalibrated their magnetic compass, but only when given full access to celestial cues, including polarized light cues near the horizon. In the current study, we carried out cue conflict experiments with white-throated sparrows, Zonotrichia albicollis, during both spring and autumn migration in a transition zone between the species' breeding and wintering areas on the south shore of Lake Ontario. We show that white-throated sparrows also recalibrate their magnetic compass by polarized light cues at sunrise and sunset. Sunrise exposure to an artificial polarization pattern shifted relative to the natural magnetic field or exposure to a shift of the magnetic field relative to the natural sky both led to recalibration of the magnetic compass, demonstrating that artificial polarizing filters do not create an anomalous, unnatural orientation response. Our results further indicate that there is no evidence for a difference in compass hierarchy between different phases of migration, confirming previous work showing that polarized light cues near the horizon at sunrise and sunset provide the primary calibration reference both in the beginning and at the end of migration.