Body size predicts the rate of contemporary morphological change in birds

Variation in evolutionary rates among species is a defining characteristic of the tree of life and may be an important predictor of species’ capacities to adapt to rapid environmental change. It is broadly assumed that generation length is an important determinant of microevolutionary rates, and body size is often used as a proxy for generation length. However, body size has myriad biological correlates that could affect evolutionary rates independently from generation length. We leverage two large, independently collected datasets on recent morphological change in birds (52 migratory species breeding in North America and 77 South American resident species) to test how body size and generation length are related to rates of contemporary morphological change. Both datasets show that birds have declined in body size and increased in wing length over the past 40 years. We found, in both systems, a consistent pattern wherein smaller species declined proportionally faster in body size and in..., For information on data collection see https://doi.org/10.5061/dryad.8pk0p2nhw and https://doi.org/10.5061/dryad.fqz612jsp.,