New study reveals small birds have wings that evolve more rapidly than those of large birds

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With vastly varying body masses, the 11,000 species of birds prove to be a remarkable system with which to decipher the potential causes behind modular evolution, according to investigators at Cornell University in Ithaca, New York.¹ A recent study conducted by Andrew Orkney, PhD, lead author and a postdoctoral researcher, and Brandon Hedrick, PhD, assistant professor in the Department of Biomedical Sciences in the College of Veterinary Medicine at Cornell University, revealed that avian wing bone size evolution is limited in heavier species of birds compared to small birds, which showed to have more freely evolving wing bones.

The investigators used a large open-access dataset comprising proportions of 228 micro-CT-scanned bird skeletons spanning various avian lineages and ecological niches and ranging in body mass from 3-g to 11-kg. Applying statistical methods and examining changes in bone proportions across a family tree constructed from publicly available phylogenies, Orkney and Hedrick assessed the impact of body mass on the evolutionary relationships within modules by categorizing birds by size and comparing modular evolution patterns. They also analyzed the co-evolution of different traits across the dataset to determine if variations between predicted trait co-evolution and observations tended to be larger or smaller in birds with different body masses.²

The findings revealed that body mass did not affect the “allied evolution” of head and leg bone sizes. However, they found compelling evidence indicating that the relative sizes of bones within the wing evolve together more closely in large birds than in small birds.² “The evolution of wing proportions is more tightly restricted in large birds, which operate under greater mechanical stresses,” Orkney said in a news release.¹

According to the investigators, this finding could also explain why small birds have developed new styles of flight, like the dynamic soaring of hyper-aerial swifts, bounding flaps of perching birds, and hovering of hummingbirds.^1,2 Because birds have "modular" skeletons, their different body parts, such as their wings, legs, and head, can evolve semi-independently.² However, in large birds, this ability is hampered due to higher levels of stress on their skeletons.

“We found that larger birds have to change different skeletal components of their wings at the same time rather than being able to evolve different parts of their wings independently,” Hedrick said in the release. “It’s easier to change a small part of the wing than all of the wing. It could be that larger animals are less capable of evolving quickly and moving into new niches more generally.”¹

In birds, this could explain the evolutionary success of small-bodied bird species. For instance, of the 11,000 bird species worldwide, 6500 belong to the category of songbirds, which are characterized by their small size.¹ “Hummingbirds are also a very successful and small group, but nothing is as successful as songbirds within birds,” Hedrick said in the release.

The study, published May 28, 2024, also found that bones in the thorax and abdomen of small birds were generally more tightly integrated. The researchers postulated that this unusual evolution in the thorax of small birds could be due to their “bounding” flight pattern, which is characterized by flapping and then propelling with wings folded.¹ “I suspect that in order to practice bounding flight, it’s important that all the proportions of the bones within that thoracic and abdominal region match each other well, explaining their tight co-evolution,” Orkney said in the release.

The study also holds potential implications for biological comprehension of animal evolution beyond avians. In forthcoming research, Orkney and Hedrick aim to explore how changes in the thorax and abdomen correlate with wing in small avian species. They will also examine whether the integration in avians differ between birds capable of flight upon hatching and those that are born helpless. Additionally, the researchers will investigate whether bats, having independently evolved flight from birds, adhere to similar evolutionary principles or if there are multiple ways to structure a flying vertebrate.¹

References

1. Wing skeleton may be less restricted in small birds. News release. Cornell Chronicle. May 28, 2024. Accessed May 28, 2024. https://news.cornell.edu/stories/2024/05/wing-skeleton-evolution-may-be-less-restricted-small-birds
2. Orkney A, Hedrick BP. Small body size is associated with increased evolutionary lability of wing skeleton proportions in birds. Nature Communications. 2024;15(1). https://doi.org/10.1038/s41467-024-48324-y