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 increased
proportionally faster in wing length. By contrast, generation length
explained less variation in evolutionary rates than did body size.
Although the mechanisms warrant further investigation, our study
demonstrates that body size is an important predictor of contemporary
variation in morphological rates of change. Given the correlations between
body size and a breadth of morphological, physiological, and ecological
traits predicted to mediate phenotypic responses to environmental change,
the relationship between body size and rates of phenotypic change should
be considered when testing hypotheses about variation in adaptive
responses to climate change.