Background: The degree to which avian migrants revisit the same sites to replicate routes from previous years has received more and more attention as the possibilities of tracking small to medium avian migrants over multiple annual cycles have improved. Repeated measurements of individuals can potentially inform about their navigation and migration strategies and to what extent observed variation within and between individuals may reflect the selective potential in the population.

Methods: We analysed the annual space-use of European nightjars (henceforth nightjars) Caprimulgus europaeus tracked with GPS-loggers in multiple years between northern Europe and southern Africa. We quantified site-fidelity to stationary sites and consistency, repeatability, and latitudinal correlations in route choice as well as investigated barrier associated changes of within- and between individual longitudinal variation in flight routes.

Results: We found that the nightjars consistently used the same breeding and wintering sites but that individual migratory route choice was flexible, although largely repeatable. Thus, rather than follow any flyway common to the study population nightjars followed individual-specific flight corridors while allowing for variation of a few hundred kilometres in route in both autumn and spring. Although the overall distribution of tracks was highly autocorrelated in both seasons we detected variations in longitudinal spread associated with the passage of ecological barriers enroute. Potential intermediate goal areas were detected prior to the crossing of the Mediterranean Sea and the Sahara Desert in both seasons. In spring, within-individual route variability dropped to a few tens of kilometres at the initiation of the desert crossing but increased to maximum over the barrier.

Conclusions: The nightjars incorporate an individual-specific space-use within their annual cycle that allow for a degree of flexibility during migration, possibly driven by the energetical benefits of allowing adaptive wind drift while airborne. Our data demonstrate how topography and spatial carry-over effects influence flight path variability that may dimmish or reinforce individuality in route choice. Hence, this study highlights that identifying and quantifying past and present external influences on behaviour and realised routes can be critical to distinguishing the genetic basis and variation in migration.

GPS sampling by dataloggers on individual birds during the annual cycle to track migration and non-breeding movements.