Start of nocturnal migratory restlessness in captive birds predicts nocturnal departure time in free-flying birds

In nocturnal songbird migrants, total speed of migration is determined by the time birds stay at stopovers, where they replenish fuel reserves used during previous flights, and by their travel speed. In contrast to factors influencing stopover duration, little is known about individual variation in travel speed which is a combination of birds' ground speed and the time spent flying. The latter is directly affected by nocturnal departure time. Ground speed can be easily tracked, e.g., by radar, but not much is known about when migrants set off within the night, let alone factors influencing its variation. Studying how factors cause variation in nocturnal departure time requires an experimental setup that allows transferring results from indoor experiments, where environmental conditions can be controlled for or manipulated, to the behavior of free-flying birds. Here, we show that the start of nocturnal migratory restlessness aEuroZugunruhe of caged songbirds was significantly positively related to their radio-tracked nocturnal departure time the following night. We can now start identifying factors causing individual variation in the start of nocturnal migratory restlessness and transfer these results to departure times within the night. This will improve our understanding of why travel speed, and with that total speed of migration, varies individually and how it is affected by environmental changes.