African departure rather than migration speed determines variation in spring arrival in pied flycatchers.

Properly timed spring migration enhances reproduction and survival. Climate change requires organisms to respond to changes such as advanced spring phenology. Pied flycatchers Ficedula hypoleuca have become a model species to study such phenological adaptations of long-distance migratory songbirds to climate change, but data on individuals' time schedules outside the breeding season are still lacking. Using light-level geolocators, we studied variation in migration schedules across the year in a pied flycatcher population in the Netherlands, which sheds light on the ability for individual adjustments in spring arrival timing to track environmental changes at their breeding grounds. We show that variation in arrival dates to breeding sites in 2014 was caused by variation in departure date from sub-Saharan Africa and not by environmental conditions encountered en route. Spring migration duration was short for all individuals, on average 2 weeks. Males migrated ahead of females in spring, while migration schedules in autumn were flexibly adjusted according to breeding duties. Individuals were therefore not consistently early or late throughout the year. In fast migrants like our Dutch pied flycatchers, advancement of arrival to climate change likely requires changes in spring departure dates. Adaptation for earlier arrival may be slowed down by harsh circumstances in winter, or years with high costs associated with early migration.