Body shrinkage due to Arctic warming reduces red knot fitness in tropical wintering range

Reductions in body size are increasingly being identified as a response to climatewarming. Here we present evidence for a case of such body shrinkage, potentially dueto malnutrition in early life. We show that an avian long-distance migrant (red knot,Calidris canutus canutus), which is experiencing globally unrivaled warming rates at itshigh-Arctic breeding grounds, produces smaller offspring with shorter bills duringsummers with early snowmelt. This has consequences half a world away at their tropicalwintering grounds, where shorter-billed individuals have reduced survival rates. This isassociated with these molluscivores eating fewer deeply buried bivalve prey and moreshallowly buried seagrass rhizomes. We suggest that seasonal migrants can experiencereduced fitness at one end of their range as a result of a changing climate at theother end