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Body sizes and shapes of birds are shrinking as the Earth warms

In a new study led by the University of Michigan, researchers have discovered that birds across the Americas are becoming smaller and developing longer wings in response to climate change. The most significant changes have been observed in the smallest-bodied species. 

This groundbreaking research, which analyzed over 86,000 bird specimens from North and South America, is published in the journal Proceedings of the National Academy of Sciences.

The researchers combined data from two previously published studies that investigated body-size and wing-length changes in migratory and nonmigratory birds. The first study focused on birds that were killed after colliding with buildings in Chicago, while the second study examined nonmigratory birds caught in the Amazon

Two studies, same results

Despite differences in species composition, geography, and data collection methods, both studies showed a widespread decline in body size and a concurrent increase in wing length.

Upon further analysis of the combined data, the researchers found an even more striking pattern: smaller bird species experienced proportionately faster declines in body size and proportionately faster increases in wing length. 

“The relationships between body size and rates of change are remarkably consistent across both datasets,” said U-M ornithologist Benjamin Winger, one of the study’s senior authors. However, Winger noted that the biological mechanism underlying this link between body size and rates of morphological change still needs to be explored further.

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 play a crucial role in species’ responses to climate change. However, the researchers are still uncertain as to why smaller-bodied species are changing at a faster rate.

What are the mechanisms behind these changes?

One possibility is that smaller-bodied birds are adapting more quickly to evolutionary pressures, although the available data did not allow the research team to test whether the observed size shifts represent rapid evolutionary changes in response to natural selection. 

Co-senior author Brian Weeks, an evolutionary ecologist at the U-M School for Environment and Sustainability, said, “If natural selection plays a role in the patterns we observed, our results suggest that smaller bird species might be evolving faster because they experience stronger selection, are more responsive to selection, or both.”

The implications of this study raise questions about the future of larger-bodied birds as global temperatures continue to rise. 

“Our results suggest that large body size could further exacerbate extinction risk by limiting the potential to adapt to rapid, ongoing anthropogenic change,” said study lead author Marketa Zimova, a former U-M Institute for Global Change Biology postdoctoral researcher now at Appalachian State University.

On the other hand, the effect of body size on evolutionary rates might increase the persistence of smaller species if their rapidly changing morphology reflects a faster adaptive response to changing environmental conditions.

How the studies were done

The experts analyzed data from 129 bird species across North and South America has found that birds are experiencing morphological changes, such as reduced body size and increased wing length, in response to climate change. The research examined 86,131 specimens collected over a similar period using different techniques, and highlights the impact of environmental changes on avian life.

Among the species studied, the smallest bird in the Chicago dataset was the golden-crowned kinglet (Regulus satrapa), averaging 5.47 grams, and the largest was the common grackle (Quiscalus quiscula) at 107.90 grams. 

In the Amazonian dataset, the fork-tailed woodnymph (Thalurania furcata) was the smallest at 4.08 grams, while the largest was the Amazonian motmot (Momotus momota) at 131.00 grams.

The North American dataset, derived from birds retrieved after collisions with buildings in Chicago, comprised 70,716 individuals. Field Museum ornithologist David Willard measured bill length, wing length, body mass, and the length of a lower leg bone called the tarsus for each specimen. 

Willard, collections manager emeritus and a co-author of the study, said, “The birds collected from window collisions in Chicago are providing insights into morphological changes related to the changing climate. It is extremely gratifying to see data from these birds analyzed for a better understanding of the factors driving these changes.”

The Amazonian dataset, on the other hand, contained measurements of 15,415 nonmigratory birds captured with mist nets in the rainforest, measured, and then released. For these birds, only mass and wing length measurements were consistently recorded throughout the study period.

Unique opportunity for the scientists

These large and complementary datasets provided researchers with a unique opportunity to test whether body size and generation length – two fundamental organismal traits – shaped birds’ responses to rapid environmental change. 

Biologists widely assume that a species’ generation length, defined as the average age of individuals producing offspring, is an important predictor of its ability to adapt to rapid environmental change.

Shorter-lived organisms that reproduce on relatively short time scales, like mice, are predicted to evolve faster than creatures with longer generation lengths, like elephants, because the former have more frequent opportunities to make use of random genetic mutations generated during reproduction.

The authors of the study used statistical models to test the importance of both 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 the North American bird species. Generation-length data were not available for the South American birds, so they were not included in that part of the analysis.

At the same time, the new analysis showed that a species’ mean body size was significantly associated with the rates of change measured in both the Chicago and Amazonian birds. 

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

More about climate change and its impact on species

Climate change is having a profound impact on both animal and plant species worldwide. As global temperatures rise, ecosystems are altered, leading to changes in species distribution, abundance, behavior, and physiology. Here are some key ways in which climate change is affecting plants and animals:

Range shifts

Many species are shifting their geographical ranges in response to climate change. Plants and animals are moving towards higher elevations and latitudes to find suitable habitats as temperatures rise. This can lead to changes in community composition, competition, and interactions between species.

Phenological changes

Phenology refers to the timing of seasonal events in the life cycle of organisms, such as flowering, migration, and reproduction. Climate change is causing shifts in phenology, with many species experiencing earlier springs, later autumns, and altered reproductive cycles. These changes can lead to mismatches between species that rely on one another, such as plants and their pollinators, or predators and their prey.

Morphological changes

As highlighted in the bird study mentioned earlier, climate change can drive changes in the physical traits of organisms. Examples include birds becoming smaller and developing longer wings, and fish species experiencing reductions in body size due to increased water temperatures.

Physiological stress

Warmer temperatures and altered precipitation patterns can cause physiological stress in plants and animals. For instance, increased water temperatures can reduce oxygen levels in aquatic habitats, leading to stress for fish and other aquatic organisms. Droughts can also stress plants, making them more susceptible to pests and diseases.

Ocean acidification

As the ocean absorbs more carbon dioxide from the atmosphere, its chemistry changes, leading to ocean acidification. This poses a significant threat to marine life, particularly species with calcium carbonate shells, such as corals, mollusks, and some plankton. Acidification can lead to reduced calcification rates, making it harder for these organisms to build and maintain their shells or skeletons.

Habitat loss and fragmentation

Climate change can exacerbate habitat loss and fragmentation by contributing to extreme weather events, sea-level rise, and altered land use patterns. As habitats become more fragmented, species may struggle to adapt or find suitable new habitats, increasing their vulnerability to extinction.

Invasive species

Climate change can facilitate the spread of invasive species by creating more favorable conditions for their establishment and growth. Invasive species can outcompete native species for resources, disrupt ecosystems, and transmit diseases.


The combination of these factors increases the risk of extinction for many plant and animal species. Climate change is expected to become a primary driver of biodiversity loss in the coming decades, with some projections estimating that up to one-third of all species could be at risk of extinction due to climate change by 2050.

These examples demonstrate that climate change has far-reaching impacts on the natural world. As a result, conservation efforts must consider the complex ways in which climate change affects ecosystems and prioritize strategies that enhance the resilience of species and ecosystems to these changes.


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