Smallest-bodied species are changing the fastest, the researchers report.

The new study combines data from two previously published papers that measured body-size and wing-length changes in a total of more than 86,000 bird specimens over four decades in North and South America. One study examined migrating birds killed after colliding with buildings in Chicago; the other looked at nonmigrating birds netted in the Amazon.

Though the two datasets are non-overlapping in both species composition and geography, and the data were collected independently using different methods, the birds in both studies displayed similarly widespread declines in body size with concurrent increases in wing length.

Now, the new analysis of the combined data has revealed an even more striking pattern: In both studies, smaller bird species declined proportionately faster in body size and increased proportionately faster in wing length.

“The relationships between body size and rates of change are remarkably consistent across both datasets. However, the biological mechanism underlying the observed link between body size and rates of morphological change requires further investigation,” says senior author Benjamin Winger, an assistant professor of ecology and evolutionary biology at the University of Michigan and an assistant curator at the Museum of Zoology.

Both the Chicago and Amazonian studies attributed the reductions in species body size to increasing temperatures over the past 40 years, suggesting that body size may be an important determinant of species responses to climate change.

Even so, exactly why smaller-bodied species are changing faster remains an open question, according to the researchers.

Read the full article about bird body sizes and climate change by Jim Erickson at Futurity.