Demand-resource mismatch explains body shrinkage in a migratory shorebird

Recent observations of body size declines in animal populations have given
rise to discussions whether or not this is related to climate
change-induced temperature increases, with which the body size changes
would follow Bergmann’s rule. Although the debate is ongoing, the limited
thermal benefits of currently observed size reductions make it unlikely
that temperature increase shapes a direct selection pressure. Food
constraints during early-life development, which could be caused by
mismatches between available resources and energetic demands, could cause
smaller body sizes too. Here we investigate whether a decrease in body
size, observed in a migratory shorebird the red knot (Calidris canutus
canutus) at their West-African non-breeding grounds over two decades, is
linked to developmental plasticity during chick growth in the High Arctic.
To do so, we combined datasets from both the wintering and breeding
grounds on body size measurements (during chick growth and in fully-grown
juveniles), food availability, and diet inferred from stable isotopes
deposited in feathers grown as chicks. From 2003 to 2021, stable-isotope
ratios revealed a decline in the dietary contribution of crane flies
(Tipulidae, Diptera), the key food of growing chicks in the Arctic. On the
breeding grounds we observed that while the emergence of adult crane flies
advanced along with earlier snowmelt dates, red knots did not adjust the
timing of breeding, and this resulted in an increasing mismatch with the
demands of growing chicks. As a result, chicks grew slower and, as
observed on the wintering grounds, reached smaller final body sizes. Our
results imply that increasing resource-demand mismatches may lead to body
shrinkage via plasticity during development. In this study the increasing
mismatch was linked with climate warming; the presented causal chain may
explain other recent examples of body size reductions as well.