Storm events will simultaneously reduce foraging opportunities and affect movements of Red Knots (Calidris canutus) in the intertidal Wadden Sea

Abstract

Intertidal systems provide important wintering areas for migratory shorebirds, where they can both forage and roost. In the light of climate change, extreme wind speeds are predicted to occur more frequently in northwestern Europe and pose a threat to shorebirds. Because strong winds can cause ‘wind set-up’ with increased water levels, increased wind speeds can affect the availability of intertidal foraging habitat of shorebirds. However, knowledge on the effects of storms on shorebirds in intertidal areas is currently limited. Using a state-of-the-art reverse-GPS system, WATLAS, we studied the effect of wind speed and direction on habitat availability and movements of Red Knots Calidris canutus islandica wintering in the intertidal Dutch Wadden Sea, a major site along the flyway. Specifically, we investigated how wind speed and direction affected maximal displacement, foraging-site selection during low tide and roost-site selection. Habitat availability was calculated by the difference in the proportion of exposed mudflat between the predicted astronomical tide and the realized tide that includes the ‘wind set-up’. Food availability was measured as the density and biomass of benthic invertebrate prey living in the exposed mudflats and are therefore potentially available to foraging shorebirds. We found a maximum decrease in mudflat availability of 50% during strong northwesterly winds and a maximum decrease in available benthic prey of 44% during strong westerly winds. Under these circumstances with strong winds, Red Knots roosted closer to the foraging grounds. However, with tailwinds (> 1 m/s) or water levels above 108 cm (Amsterdam Ordnance Datum) birds nonetheless flew to a more distant roost. We conclude that strong westerly winds limit mudflat and food availability for Red Knots and affect movement costs through foraging and roosting decisions. These wind-driven effects are likely to increase energetic costs while reducing opportunities for food intake, thereby reshaping energy balances. The predicted increase in strong westerly winds with the climate crises could intensify these effects, leading to more frequent and prolonged reductions in foraging opportunities, and probably posing growing challenges for shorebirds in this important UNESCO World Heritage Site and globally in intertidal areas.