A University of Michigan-led study of approximately 70,000 North American migratory birds from 52 species found that body sizes have decreased and wing length has increased in all of the species since 1978.
David Willard, a Field Museum ornithologist, has measured the Windy City's dead birds since 1978. Data from his calipers and scales reveal decades-long trends in bird bodies: Their legs, on average, are growing shorter. They have lost weight. Their wings are getting slightly longer.
These changes are present in nearly all of the species he measured, according to a study of 70,716 bird specimens from almost 40 years published Wednesday in the journal Ecology Letters. Morphing birds, Willard, and his colleagues say, reflect a changing climate: "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".