Shared morphological consequences of global warming in North American migratory birds

Increasing temperatures associated with climate change are predicted to
cause reductions in body size, a key determinant of animal physiology and
ecology. Using a four‐decade specimen series of 70 716
individuals of 52 North American migratory bird species, we demonstrate
that increasing annual summer temperature over the 40‐year period predicts
consistent reductions in body size across these diverse taxa.
Concurrently, wing length – an index of body shape that impacts numerous
aspects of avian ecology and behaviour – has consistently increased across
species. Our findings suggest that warming‐induced body size reduction is
a general response to climate change, and reveal a similarly consistent
and unexpected shift in body shape. We hypothesise that increasing wing
length represents a compensatory adaptation to maintain migration as
reductions in body size have increased the metabolic cost of flight. An
improved understanding of warming‐induced morphological changes is
important for predicting biotic responses to global change.