Long-term increases in wing length occur independently of changes in climate and climate-driven shifts in body size.

Recent widespread reductions in body size across species have been linked to increasing temperatures; simultaneous increases in wing length relative to body size have been broadly observed but remain unexplained. Size and shape may change independently of one another, or these morphological shifts may be linked, with body size mediating or directly driving the degree to which shape changes. Using hierarchical Bayesian models and a morphological time series of 27 366 specimens from five North American migratory passerine bird species, we tested the roles that climate and body size have played in shifting wing length allometry over four decades. We found that colder temperatures and reduced precipitation during the first year of life were associated with increases in wing length relative to body size but did not explain long-term increases in wing length. We found no conclusive evidence that the slope of the relationship between body size and wing length changed among adult birds in response to any climatic variable or through time, suggesting that body size does not mediate shifts in relative wing length. Together, these findings suggest that long-term increases in wing length are not a compensatory adaptation mediated by size reductions, but rather are driven by non-climatic factors.