| Summary: | Animal body shrinkage appears correlated with climate warming, but the mechanism remains unclear. For an Arctic-breeding shorebird, the red knot, we demonstrate why juvenile body size at its West-African non-breeding grounds has decreased over two decades. Over this period, stable-isotope ratios sampled from juvenile feathers - grown as chicks on their Arctic breeding grounds 9,000 kilometers away - reveal a decline in the dietary contribution of crane flies, their key food source on the tundra. With crane fly phenology advancing with earlier snowmelt dates but red knot breeding timing not, this has caused an increasing mismatch with the demands of growing chicks, leading to slower growth and smaller final body sizes. Our results imply that body shrinkage may come about rapidly via plasticity during development.
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