Foraging ecology and diet of Eurasian spoonbills (Platalea leucorodia) in the German Wadden Sea

Eurasian spoonbills (Platalea leucorodia) were first recorded breeding in the German Wadden Sea in 1995. Since then, the breeding numbers have been increasing. However, the foraging habitats and diet of spoonbills in this area remain largely unknown. We therefore analysed the foraging ecology of spoonbills from three colonies in this area using GPS telemetry and examined their diet based on regurgitates and stable isotope analysis. GPS logger data (2014–2016) implied that tidal creeks near the mainland coast are used as the main foraging hotspots in the breeding season. Regurgitates (n = 77) and blood samples (n = 78) collected from spoonbill chicks in 2014 and 2016 showed slight variations in diet compositions between years and among colonies. Regurgitates (reflecting the diet during the day prior to sampling) included prey items ranging from ragworms and shrimps to fish, indicating an opportunistic foraging strategy. Smelts (31.9%) (Osmerus eperlanus), sticklebacks (22.6%) (Gasterosteus aculeatus), and gobies (13.3%) (Pomatoschistus minutus) were the dominant prey organisms by mass. Analyses of δ15N and δ13C values in blood serum of spoonbill chicks (reflecting the diet 1 week before sampling) showed a diverse diet composition, with flatfish, smelts, gobies, and sticklebacks as the main prey species at the end of May. In contrast, values in red blood cells (reflecting the diet up to 3 weeks prior to sampling) demonstrated that sticklebacks and flatfish were the main dietary components of spoonbills in the German Wadden Sea in early May. The combination of different diet analyses determined the diet choice of spoonbills in the German Wadden Sea on different time scales. Telemetry data enabled the study of fine-scale habitat use.