Phenotypic response to environmental cues, orientation and migration costs in songbirds flying halfway around the world

Migrantsʹ responses to varying meteorological and magnetic conditions and their consequent costs in terms of energy and time are crucial for the understanding of the spatial principles guiding migration. Until now, studies of birdsʹ phenotypic responses to environmental conditions and estimated costs of migration in terms of energy and time have focused on single sites and have rarely followed individuals along entire migration routes. We used individual-based light level geolocators to track birdsʹ daily phenotypic responses to the environmental conditions experienced along a 30 000 km migration between Alaska and Africa. In autumn, departures of northern wheatears, Oenanthe oenanthe, were highly dependent on low temperatures and light winds but there was no such apparent dependence in spring. Migration was 1.4-times faster in spring than in autumn. There was an unexpected shift in the ratio of migration:stopover time towards less stopover time in spring, reducing springʹs total energy costs for migration with respect to distance covered by nearly 50% in comparison to autumn. Despite strong changes in declination during migration, offering near-experimental conditions, birds did not navigate exclusively along any major compass course. In contrast to expectations, these high-Arctic migrants did not follow great circle routes; their hypothetical energetic benefit (ca. 20% for flying) might be insufficient and conditions for orientation too unfavourable to favour its evolution.