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.

NOTE: Tarsus length was measured by holding tarsus at a right angle to tiobiotarsus, and measuring form the heel to the end of the last scute before the foot; this adds the length of the joint, so absolute measures of tarsus length are likely not directly comparable to tarsus length estimates from other studies. All measurements were made consistently across species and time (i.e. they are internally consistent and replicable). 

NOTE: Bill length, from the anterior edge of the nares to the tip of the bill has been added to the data.