Geographic variation in the diet of Red Knot (Calidris canutus) chicks reflects local prey communities

Rapid climate-driven alterations to Arctic ecosystems threaten the phenological synchrony between migratory shorebirds and their invertebrate prey. Assessing the adaptive capacity of these species requires a thorough understanding of dietary plasticity across their breeding ranges. Here, we characterise the diet of Red Knot (Calidris canutus) chicks across three geographically distinct breeding grounds, Northwest Alaska (Nome), Northeast Greenland (Zackenberg) and North-Central Russia (Taimyr Peninsula), by combining COI DNA metabarcoding of faecal samples with standardised pitfall trapping assessment of local arthropod prey availability. We report substantial geographic variation in foraging strategies driven by the local prey community structure. In Taimyr, the chick diet was highly specialised and dominated by Tipulidae (crane flies) and Chironomidae, tracking the high local biomass of these taxa. Conversely, Alaskan populations exhibited a broad, generalist diet comprising Diptera, Coleoptera, Lepidoptera, and Gastropoda, reflecting a diverse prey landscape with low abundance of Tipulidae. Chicks in Greenland displayed an intermediate strategy, specialising in Diptera but virtually excluding Tipulidae. These findings demonstrate that Red Knot chicks are not intrinsic specialists, but rather possess significant foraging plasticity, enabling them to exploit heterogeneous prey communities. However, the distinct reliance on phenologically constrained Tipulidae in Taimyr highlights the critical vulnerability to potential trophic mismatches in the fastest-warming region of the Arctic.