Population-specific assessment of carry-over effects across the range of a migratory songbird

Annual cycle events may be interlinked, influence following annual cycle stages and may alter performance of individuals. Such links, called carry-over effects, can explain individual variation in timing or reproductive success in migratory species. Identifying the key links affecting fitness may reveal mechanisms of species population dynamics but the current evidence for the strongest carry-over effects is equivocal. Here, we aim at assessing carry-over effects in great reed warblers Acrocephalus arundinaceus, a long-distance migratory songbird, using 103 full-annual tracks from three European and two Asian breeding populations. Our results showed strong positive relationships within autumn and spring migration periods and buffering capacity of the non-breeding period preventing events to carry over between these periods. Moreover, we found no profound relation between the non-breeding habitat quality or seasonality (quantified using stable isotopes and remote sensing data) and the timing of spring migration. The strongest carry-over effects occurred in individuals from the southern European breeding population compared to the northern and the central European populations. A moderate relationship between the habitat seasonality during moult and the spring migration timing indicates the importance of the complete moult. The overall weak carry-over effects of non-breeding habitat conditions found in this study contrast with previous results and imply between-species differences in these crucial relationships. Moreover, the population-specific carry-over effects highlight the importance of multi-population approach and advise caution in interpretation of results from single-population studies. Finally, the carry-over effect from the moulting period indicates significance of a so far neglected link in the species.

The database consists of two datasets: dataset_carry-over_grw_zenodo.csv containing data used for the calcultion of the carry-over effects within the annual cycle and feather_variability_grw_zenodo.csv containing stable isiotopic values of three feather types sampled from individuals breeding in Sweden, Czech Republic and Bulgaria.