Circumpolar variation in Red Knot chick diets: selectivity is shaped by crane fly abundance

Dietary specialisation is a key ecological trait shaping predator-prey interactions and evolutionary adaptations in vertebrates. We investigated the dietary selectivity patterns of three Red Knot (Calidris canutus) subspecies breeding across the Arctic, in Northwest Alaska, Northeast Greenland and North-Central Russia, using standardised arthropod sampling and next-generation sequencing of faecal samples. Our results revealed substantial differences in the estimated dietary profiles of Red Knot chicks, with Diptera emerging as the dominant prey group. In North-Central Russia, Tipulidae (crane flies) dominated the chick diets, constituting nearly 50% of the mean dietary composition. Conversely, Greenland chick diets showed a near absence of Tipulidae, while diets from Alaska exhibited a balanced contribution from Coleoptera, Lepidoptera, and Diptera. Analyses of dietary selectivity revealed a strong specialisation in North-Central Russia and a more moderate and opportunistic selectivity in Alaska and Greenland. When using Tipulidae as a proxy for ‘high-value prey’, due to its relatively high abundance, biomass, and limited manoeuvrability, we observed a notable inverse relationship between dietary diversity and Tipulidae biomass availability across breeding localities. Our findings demonstrate that the estimated dietary profiles of Red Knot chicks reflect regional ecological conditions, with the arthropod community structure playing a key role. This study advances our understanding of Arctic trophic dynamics and the potential vulnerabilities of migratory bird populations in a rapidly changing Arctic.