Abstract
Shorebird populations, especially those feeding on shellfish, have strongly declined in recent decades and identifying the drivers of these declines is important for conservation. Changing food stocks are thought to be a key driver of these declines and may also explain why trends have not been uniform across Europe's largest estuary. We therefore investigated how winter population trends of Eurasian oystercatchers (Haematopus ostralegus) were linked to food availability in the Dutch Wadden Sea. Our analysis incorporated two spatial scales, a smaller scale focused on roost counting areas and food available to birds in these areas and a larger spatial scale of tidal basins. A novelty in our study is that we quantify the connectivity between roosting and foraging areas, identified from GPS tracking data. This allowed us to estimate food available to roosting birds and thus how food availability may explain local population trends. At the smaller spatial scale of roost counting areas, there was no clear relationship between available food and the number of roosting oystercatchers, indicating that other factors may drive population fluctuations at finer spatial scales. At the scale of tidal basins, however, there was a significant relationship between population trends and available food, especially cockle Cerastoderma edule,. Mortality and recruitment alone could not account for the large fluctuations in bird counts, suggesting that the site choice of wintering migratory oystercatchers may primarily drive these large fluctuations. Furthermore, the relationship between oystercatcher abundance and benthic food stocks, suggests winter shorebird counts could act as ecological indicators of ecosystem health, informing about the winter status of food stocks at a spatial scale of tidal basins.
Highlights
Intertidal areas support a rich benthic fauna across the world and are a vital habitat for staging, breeding and wintering water birds (Hua et al, 2015; Blew et al, 2017)
Our analysis focused on the winter period, which is an energy demanding period when oystercatchers largely depend on the intertidal areas for foraging (Goss-Custard et al, 1996a; van der Kolk et al, 2020), and the oystercatcher population in the Wadden Sea drastically in creases with wintering migrants, meaning that competition for food is greater and competition-sensitive species like the oystercatcher can be expected to distribute according to avail able resources (Goss-Custard et al, 1996b; Folmer et al, 2010)
Fifty-eight roosting polygons and 140 foraging polygons were iden tified from the Global Positioning System (GPS) data
Summary
Intertidal areas support a rich benthic fauna across the world and are a vital habitat for staging, breeding and wintering (migratory) water birds (Hua et al, 2015; Blew et al, 2017). Studies have shown that over-winter survival is strongly dependent on available food stocks, especially during periods of severe weather (Camphuysen et al, 1996; Atkinson et al, 2003). Many waterbirds that utilise intertidal areas are highly mobile and local declines of benthic prey may result in predator redistribution patterns to other foraging areas (Atkinson et al, 2003), as exemplified by how the winter distribution patterns of Common Eider (Somateria mollissima) appear to be linked to available food stocks (Cervencl et al, 2015). Studies that relate shorebird populations to benthic fauna should consider benthic fauna at local sites, and benthic stocks available in the wider landscape
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