The spatial consistency of migratory route and stopover choice in European nightjars quantified by nearest neighbour analysis on multi-annual GPS tracks

Abstract

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.

Assessing an age-old ecogeographical rule in nightjars across the full annual cycle

“…If an individual was captured during multiple years, we discarded data from when individuals were juveniles, and averaged morphological data from all adult capture occasions. All individuals tracked across multiple years in our study wintered within a few hundred meters of previous years (16/16 of whip-poor-will, 7/7 European nightjars; Bakermans et al, 2022;Norevik et al, 2022a;Skinner et al, 2022b;Korpach et al, in prep), thus we used the winter location from the first year available. The majority of individuals caught across years on the breeding grounds were captured in the same location, and all individuals were captured within 2 km of the original capture location (Ng et al, 2018;Norevik et al, 2022a).…”

“…However, the mismatch we observed between morphology and environmental conditions experienced on the wintering grounds is concerning and may be part of the reason these three species are declining. Climate change may further exacerbate this mismatch, particularly given that Caprimulgids appear to have high site fidelity across the annual cycle (Ng et al, 2018;Bakermans et al, 2022;Norevik et al, 2022b) and long lifespans (> 10 year), resulting in a lag between range shifts and climate shifts. Climate change appears to most strongly impact long-distance migrants that exhibit low diversity in migratory strategy (e.g., obligate migrants, small non-breeding ranges; Both et al, 2010;Gilroy et al, 2016), as is the case of the three Caprimulgids examined in this study.…”