After an historical absence, over the last decades Eurasian Spoonbills Platalea leucorodia leucorodia have returned to breedon the barrier islands of the Wadden Sea. The area offers an abundance of predator-free nesting habitat, low degrees ofdisturbance, and an extensive intertidal feeding area with increasing stocks of brown shrimp Crangon crangon , the assumedmain prey of P. leucorodia leucorodia. Nevertheless, newly established and expanding colonies of spoonbills have surprisinglyquickly reached plateau levels. Here we tested the often stated assertion that spoonbills mainly rely on brown shrimpas food, by quantifying the diet of chicks on the basis of regurgitates and by analysis of blood isotopes using stable isotopeBayesian mixing models. Both methods showed that, rather than brown shrimp being the staple food of spoonbill chicks,small flatfish (especially plaice Pleuronectes platessa ) and gobies ( Pomatoschistus spp.) were their main prey. Unlike shrimp,small flatfish have been reported to be rather scarce in the Wadden Sea in recent years, which may explain the rapid saturationof colony size due to food-related density-dependent recruitment declines of growing colonies. By way of their diet andcolony growth characteristics, spoonbills may thus indicate the availability of small fish in the Wadden Sea. We predict thatthe recovery to former densities of young flatfish and other juvenile/small fish in the Wadden Sea will be tracked by changingdiets (more fish) and an increase in the size of Eurasian Spoonbill colonies across the Wadden Sea.