“Body size appears to be a primary mediator of birds’ responses to contemporary climate change.”

A new University of Michigan study has revealed that birds across the Americas are experiencing significant changes in size and wing shape due to climate change, with the smallest birds undergoing the most dramatic transformations.

According to a new study led by the University of Michigan, as the world warms, birds throughout the Americas are becoming smaller and longer-winged, with the smallest species showing the greatest changes.

The research, scheduled for online publication in the Proceedings of the National Academy of Sciences today, combines data from two previously published studies, which examined body size and wing length changes in over 86,000 bird specimens across North and South America. One study focused on migrating birds in Chicago, while the other examined non-migrating birds in the Amazon.

Despite the studies having different species and geographical compositions and employing different collection methods, they found similar widespread declines in body size, accompanied by increases in wing length.

The new analysis of the combined data shows that smaller bird species have declined proportionately faster in body size while increasing proportionately faster in wing length in both studies.

“The relationships between body size and rates of change,” according to U-M ornithologist Benjamin Winger, one of the study’s two senior authors, “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.”

According to studies conducted in Chicago and the Amazon, the decrease in the size of species is due to rising temperatures over the last four decades, indicating that climate change may have a significant impact on body size.

However, researchers have yet to determine why smaller species are changing more rapidly.

According to the U-M-led team, it is possible that smaller-bodied birds are adapting faster to evolutionary pressures. However, due to the limited data available, they were unable to verify whether the observed changes in size are a result of rapid evolutionary modifications in response to natural selection.

“If natural selection plays a role in the patterns we observed,” adds co-senior author Brian Weeks, the findings “suggest that smaller bird species might be evolving faster because they experience stronger selection, are more responsive to selection, or both.

It seems that the size of a bird’s body plays a significant role in how they react to the effects of climate change in the present day.

With larger birds exhibiting a slower response to global changes, the question arises as to what lies ahead in the coming decades as temperatures continue to rise.

The results of this study show “that large body size could further exacerbate extinction risk by limiting the potential to adapt to rapid, ongoing anthropogenic change,” points out study lead author Marketa Zimova.

“In contrast, the body-size effect on evolutionary rates might increase persistence of small taxa if their rapidly changing morphology reflects a faster adaptive response to changing conditions.”

The new study examined data from 129 bird species, comprising 52 migratory species that breed in North America and 77 resident species in South America. Over a period of time, 86,131 specimens were collected using different techniques.

The study found that the golden-crowned kinglet was the smallest bird among the Chicago species, with an average size of 5.47 grams, while the common grackle was the largest at 107.90 grams. Among Amazonian species, the fork-tailed woodnymph was the smallest at 4.08 grams, and the Amazonian motmot was the largest at 131.00 grams.

The North American dataset used in the study was collected from birds retrieved by staff and volunteers at Chicago’s Field Museum, following collisions with city buildings. The museum’s ornithologist, David Willard, measured several aspects of each of the 70,716 individuals, including bill length, wing length, body mass, and the length of the tarsus bone.

“The birds collected from window collisions in Chicago are providing insights into morphological changes related to the changing climate,” adds Willard, one of the co-authors of the PNAS Study.

“It is extremely gratifying to see data from these birds analyzed for a better understanding of the factors driving these changes.”

The dataset gathered from the Amazon rainforest comprises data on 15,415 nonmigratory birds that were captured using mist nets, measured, and subsequently released back into their habitat. The data includes two crucial measurements that were consistently recorded over the entire study period – the birds’ mass and wing length. This comprehensive dataset provides valuable insights into the characteristics and behavior of nonmigratory bird populations in the Amazonian region.

The study aimed to test whether two fundamental traits of organisms, body size and generation length, played a role in shaping the birds’ responses to rapid environmental changes. It is widely believed among biologists that a species’ generation length, which refers to the average age of individuals producing offspring, is an essential factor in predicting its ability to adapt to environmental changes quickly.

The authors of the study used statistical models to investigate the importance of both generation length and body size in mediating rates of morphological change in birds. After accounting for body size, they found no link between generation length and rates of change in North American bird species. Unfortunately, data on generation length for South American birds were not available for inclusion in the analysis.

However, the study did demonstrate a significant association between a species’ mean body size and the rates of change measured in both the Chicago and Amazonian birds. The large and complementary datasets obtained from the Amazon rainforest presented researchers with a unique opportunity to gain insights into the impacts of environmental changes on nonmigratory bird species.

“Body size may be a valuable predictor of adaptive capacity and the extent to which contemporary evolution may reduce risk of extinction among species,” according to the authors.

The authors of the PNAS paper include Winger, Weeks, Zimova, Willard, Giery from Pennsylvania State University, Jirinec from Integral Ecology Research Center, and Burner from the U.S. Geological Survey.

The study received financial support from the Institute for Global Change Biology at the U-M School for Environment and Sustainability.

Image Credit: Stacey Wescott/Chicago Tribune/Tribune News Service via Getty Images