Birds across the Americas are getting smaller and longer-winged as the world warms, and smaller species are changing size faster.
This is the key finding of a new study led by the University of Michigan and scheduled to be published online May 8 in the journal Nature Proceedings of the National Academy of Sciences.
The study brings together data from two previously published papers and measured changes in body size and wing length in more than 86,000 bird samples over four decades in North and South America. One study examined migratory birds killed after crashing into buildings in Chicago. The other looked at non-migratory birds confined to the Amazon.
Although the two datasets are not overlapping in both species composition and geography, and the data were collected independently using different methods, the birds in both studies showed widespread decreases in body size with simultaneous increases in wing length.
Now, a new analysis of the pooled data has revealed an even more impressive pattern: In both studies, smaller bird species decreased relatively faster in body size and increased relatively faster in wing length.
“The relationships between body size and rates of change are remarkably consistent across the two data sets. However, the biological mechanism underlying the observed association between body size and rates of morphological change requires further investigation,” said ornithologist Benjamin Winger, one of two study authors. Senior authors, assistant professor of ecology and evolutionary biology, and assistant curator at the Museum of Zoology.
Both the Chicago and Amazon studies attributed declines 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.
However, why smaller species change size faster remains an open question, according to the researchers.
Smaller birds may have adapted more quickly to evolutionary pressures. But the available data did not allow the team led by UM to test whether the observed size shifts represented rapid evolutionary changes in response to natural selection.
“If natural selection plays a role in the patterns we observed, our results suggest that smaller bird species may evolve faster because they experience stronger selection, more responsive selection, or both,” said co-lead author Brian Weeks. Environmental scientist at the UM School of Environment and Sustainability.
Either way, body size appears to be the primary mediator of birds’ responses to contemporary climate change.
So if larger birds are responding more slowly to global change, what is the prediction for the coming decades, as temperatures continue to rise?
“Our findings suggest that large body size could exacerbate extinction risk by reducing the adaptive potential for rapid and ongoing human changes,” said study lead author Marketa Zimova, a former postdoctoral researcher at the UM Institute for Biology of Global Change now in Appalachian State University.
“In contrast, the influence of body size on evolutionary rates may increase the persistence of small taxa if their rapidly changing shape reflects a faster adaptive response to changing conditions.”
The new study analyzed data from 129 bird species: 52 migratory species that breed in North America and 77 species that are resident in South America. 86,131 samples were collected during approximately the same period using different techniques.
The smallest bird among the Chicago species was the golden-crowned king (Regulus satrapa) at an average size of 5.47 grams, and the largest was the common warbler (Quiscalus quiscula) at 107.90 grams. Among the Amazonian species, the spiny-tailed woody nymph (Thalurania furcata) was the smallest at 4.08 g, and the largest was the Amazonian (Momotus momota) at 131.00 g.
The North American data set was derived from birds that were recovered by staff and volunteers at the Field Museum in Chicago after crashing into city buildings. Field Museum ornithologist David Willard measured the beak length, wing length, body mass, and length of a lower leg bone called the tarsus for each of the 70,716 individuals.
“The birds collected from window collisions in Chicago provide insights into morphological changes related to climate change. It is gratifying to see the data from these birds analyzed in order to better understand the factors driving these changes,” said Willard, collections director emeritus. . and co-author of the new PNAS study.
The Amazonian dataset contains measurements of 15,415 non-migratory birds that were captured with mist nets in the rainforest, measured, and then released. Two measurements were recorded consistently throughout the study: mass and wing length.
The large, complementary datasets provided a unique opportunity to test whether two fundamental traits of organisms–body size and generation length–shaped avian responses to rapid environmental change.
Among biologists, it is widely assumed that a species’ generation length, defined as the average lifespan of individuals that produce offspring, is an important indicator of its ability to adapt to rapid environmental change.
Short-lived organisms that reproduce over relatively short time scales, such as mice, can be expected to evolve faster than organisms with longer generations, such as elephants, because mice have more frequent opportunities to take advantage of random genetic mutations generated during reproduction. .
The authors of the new PNAS study used statistical models to test the importance of generation length and species body size in mediating rates of morphological change in birds.
After controlling for body size, they found no relationship between generation length and rates of change in North American bird species. Generation length data was not available for South American birds, so it was not included in this part of the analysis.
At the same time, the new analysis showed that the species’ average body size was significantly related to the rates of change measured in both Chicago and Amazon birds.
“Body size may be a valuable predictor of adaptive capacity and the extent to which contemporary evolution may reduce the risk of extinction among species,” the authors wrote.
In addition to Weeks, Weeks, Zimova and Willard, the authors of the PNAS paper are Penn State University’s Sean Giery, Vitek Jirinec of the Integrated Environment Research Center and Ryan Burner of the USGS.
The study was supported by funding from the Institute for Global Change Biology at the UM School of Environment and Sustainability.