The degree to which avian migrants return to the same stationary sites to mimic routes from previous years has received more and more attention as the possibility of tracking small to medium avian migrants over multiple annual cycles has increased. Repeated measurements of individuals can potentially inform about their navigation and migration strategies and to what extent the degree of variation observed within and among individuals may reflect the selective potential in the population. Here we perform a k-nearest neighbour analysis along with a repeatability measure to distinguish events in the annual cycle with intra-individual spatial convergence and to quantify the degree of individual consistency and repeatability at those events. To demonstrate the usefulness of our approach we analyse the annual space-use of European nightjars (henceforth nightjars) Caprimulgus europaeus tracked in multiple years between northern Europe and southern Africa. We found that the nightjars consistently used the same breeding and wintering sites but that individual route choice during migration were flexible, but significantly repeatable relative to population level variation during the Sahara-crossing. Thus, the nightjars followed individual-specific flyways while allowing for variation of a few hundred kilometres in the actual route in both autumn and spring. The exception was a strong within-individual convergence down to a few tens of kilometres recorded at the initiation of the trans-Saharan flight in spring. Our results suggest that nightjars have incorporated an individual-specific space-use within their annual cycle, but that they allow for a state-dependent flexibility possibly driven by the cost-benefit balance between the use of known stationary sites and an economical route-choice.

Understanding the trade-off between energy expenditure of carrying large fuel loads and the risk of fuel depletion is imperative to understand the evolution of flight strategies during long-distance animal migration. Global flyways regularly involve sea-crossings that may impose flight prolongations on migrating land-birds and thereby reduce their energy reserves and survival prospects. We studied route choice, flight behavior and fuel store dynamics of nocturnally migrating European nightjars (Caprimulgus europaeus) crossing water barriers. We show that barrier size and airspeed of the birds influence the prospects of extended daylight flights, but also that waters possible to cross within a night regularly results in diurnal flight events. The nightjars systematically responded to daylight flights by descending to about a wingspan’s altitude above the sea-surface while switching to an energy-efficient flap-glide flight-style. By operating within the surface – air boundary layer, the nightjars could fly in ground effect, exploit local updraft and pressure variations, and thereby substantially reduce flight costs as indicated by their increased proportion of cheap glides. We propose that surface-skimming flights, as illustrated in the nightjar, provide an energy-efficient transport mode and that this novel finding asks for a reconsideration of our understanding of flight strategies when land-birds migrate across seas.