Annual and spatial variation in adult and brood abundance in a sub-Arctic wader

Capsule: Numbers of adult Eurasian Whimbrels Numenius phaeopus declined steeply across Iceland’s largest lowland basin during 2012–2021, but the abundance of broods during 2013–2021 did not decline. Sites with more adults generally held more broods. Aims: Many Arctic-breeding waders are experiencing population declines, and it is imperative to gain a better understanding of how habitats and demography interplay in driving the dynamics of wader populations. We investigated the temporal and spatial variation in the abundance of Eurasian Whimbrel adults and broods (a proxy for productivity), and also the habitat associations of adults and broods, to assess whether sites holding more adults were also more productive. Methods: Temporal and spatial variation in the abundance of Eurasian Whimbrels were analysed using generalized linear models in an 11-year dataset of adult counts and a nine-year dataset of brood counts, collected in a road-based survey across Iceland’s largest lowland region. Results: We found a strong decline in the adult population but no trend in brood abundance, which showed high annual variation. Both the adults and broods occurred in a range of semi-natural habitats, but broods occurred less frequently in cultivated, built-up and forested habitats than expected by chance. More broods were associated with survey points that had a higher abundance of adults, suggesting the sites holding more adults are also more productive. Conclusions: Spatial variation in brood abundance is largely a proximate product of local adult abundance, whereas its temporal variation seems strongly affected by annual environmental conditions. Incorporating such results into landscape planning, and prioritizing habitats for conservation where adults are more abundant, should also favour more productive sites in this Icelandic population.