Birds are changing their body size and wing length as the world warms and the smallest species are changing the fastest, according to a new study by the University of Michigan researchers.
The study, published online on May 8 in the journal Proceedings of the National Academy of Sciences, combines data from two previous studies that measured changes in more than 86,000 bird specimens over four decades in North and South America.
The researchers found that both migratory and nonmigratory birds across the Americas displayed widespread declines in body size with concurrent increases in wing length.
They also found that smaller bird species declined proportionately faster in body size and increased proportionately faster in wing length than larger species.
Why are birds changing their size and shape?
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The researchers attributed the reductions in bird 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, as per Phys.org.
One possible explanation is that smaller birds have a higher surface-area-to-volume ratio, which allows them to dissipate heat more efficiently.
Another possibility is that smaller birds have lower metabolic rates, which reduces their water and energy requirements.
The increases in wing length may reflect an adaptation to increased air turbulence caused by climate change.
Longer wings may help birds fly more efficiently and maneuver better in windy conditions.
However, the exact mechanisms underlying the observed link between body size and rates of morphological change require further investigation, according to the researchers.
The study suggests that smaller bird species may be evolving faster than larger ones in response to climate change, either because they experience stronger selection pressures or because they are more responsive to selection, as per ScienceDaily.
This could have positive or negative consequences for bird conservation, depending on the context.
"On one hand, the body-size effect on evolutionary rates might decrease persistence of large taxa if their slowly changing morphology reflects a slower adaptive response to changing conditions," said co-senior author Brian Weeks, an evolutionary ecologist at the U-M School for Environment and Sustainability.
On the other hand, 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 study also highlights the importance of museum collections as sources of data for studying long-term biological changes.
"The specimens we analyzed were collected over many decades by hundreds of different people, mostly working independently," said co-senior author Benjamin Winger, an ornithologist and assistant professor of ecology and evolutionary biology at U-M.
Without their efforts and the foresight of museum curators who preserved and organized these specimens, this study would not have been possible.
The study analyzed data from 129 bird species: 52 migratory species breeding in North America and 77 South American resident species.
The data were derived from two previously published papers: one by Weeks et al. (2019) on Chicago collision victims and one by Winger et al. (2020) on Amazonian birds.
