Wetenschappelijk artikelRegional-scale responses of shorebird numbers to benthic food resources in the Dutch Wadden Sea: contrasts between six species

The UNESCO Wadden Sea is the world's largest intertidal ecosystem, recognized as a site of major importance for migratory shorebirds. Long- and short-term changes in shorebird numbers have been attributed to changes in food resources, but this relationship remains poorly tested, particularly at larger spatial and temporal scales. Here, we combined spatially and temporally comprehensive counts of the six most abundant shorebird species with annual benthic resource surveys, conducted throughout the Dutch Wadden Sea between 2008 and 2023. We examined (i) temporal trends in shorebird numbers and food resources, (ii) whether food resources potentially limited shorebird numbers, and (iii) whether shorebird numbers correlated with food resources. Over the study period, numbers of oystercatcher Haematopus ostralegus and curlew Numenius arquata declined, while dunlin Calidris alpina and grey plover Pluvialis squatarola numbers increased. Red knot Calidris canutus numbers fluctuated, and bar-tailed godwits Limosa lapponica numbers remained stable. Food availability fluctuated substantially among years, but none of the benthic invertebrate species exhibited a significant long-term increase or decrease. Prey densities suggest that in the Wadden Sea numbers of most shorebirds were unlikely to be sustained by a single prey species and instead depended on multiple prey types. Only red knot numbers correlated with the available biomass of their preferred prey. Our results therefore suggest that, for most shorebirds, changes in food resources within the Wadden Sea were unlikely to be the primary cause of numerical changes. We discuss how shorebird numbers may be regulated at multiple sites and outline future research approaches.