The physiology of birds is conducive to flight: small size, hollow bones and generally symmetrical feathers on the wings and tail. It seems obvious that bird evolution has been optimized for flight, but a new study suggests things are not as clear-cut as they seem.
The conclusion of a recent study in Nature Communications It sounds pretty unintuitive: most bird wings don’t seem to be built for maximum flight efficiency and aren’t perfectly tuned for the way birds fly. What’s more, the birds with the most optimized wing shapes in the study were hummingbirds and penguins; the latter used their wings to swim skillfully, not to fly through the air. The researchers analyzed 1,139 images of bird wings using a method that allowed them to test the relative performance of different wing shapes without assumptions about what constituted an “optimal” wing shape.
“For many birds, the functional limitations of flight (the need to generate lift, overcome drag, and turn strongly) do not greatly influence wing shape.” Benton Waltersa doctoral researcher at the University of Bristol in the United Kingdom told Gizmodo. “For some groups, these pressures have resulted in the evolution of optimal wing shapes, but this is the exception rather than the rule.”
Two wings and a prayer.
There is a surprisingly active and heated debate about why some animals evolved to fly. The discourse dates back to the 1880s, but the hypotheses are not testable, making it a “useless debate,” according to the late ornithologist John Hutchinson in a blog post for the University of California Museum of Paleontology. So, Hutchinson added, researchers like to focus on what characteristics enable flight. For modern birds, wings are the “structures that support flight,” the article explains, and birds have “adopted a wide range of flight styles and great variability in wing shape.”
But the deeper researchers delved into this diversity, the less sure they were about the evolutionary path that led to the wings of modern birds.
Wings are made… to fly?
Walters and his colleagues sought to test the traditional, and possibly intuitive, assumption that bird wings evolved to be better optimized for flight. Simply, natural selection It dictates that being better adapted to the environment and lifestyle increases an animal’s chances of survival. If flight helps birds evade predators or cold weather and find better food, wouldn’t there be a natural incentive for wings to become the best versions of themselves?
“This is called adaptationist thinking,” Walters told Gizmodo. “Given that flying is a demanding mode of travel and has extensively changed the body plan of birds over the roughly 100 million years they have existed, birds should have evolved optimally shaped wings.”
Define “optimal”
But it is typically “very difficult, if not impossible, to determine whether an animal is optimal, because there is no way to know, by looking only at live animals, whether the best observed form is the best overall,” Walters said. To overcome this challenge, the team turned to theoretical morphospatial analysis. This method identifies the best evolved shape for a particular flight style, rather than assuming that a bird’s current wing shape is the most optimal.
The study analyzed 1,139 images of bird wings and also considered “specialized” flight styles such as dynamic flight and flight styles of species that migrate long distances, Walters said. As expected, flightless birds like ostriches had wings that were “pretty poor,” he added.
But the analysis found bird species whose empirical wing shape closely resembled the predicted optimal wing shape, namely members of the hummingbird and penguin families. Birds with high energy-demanding flight styles, such as aerial hawking, also tended to have better optimization. As expected, flightless birds like ostriches had wings that were “pretty poor,” Walters explained.
Evolution is complicated
Generally speaking, though, it seemed that for most birds “good enough is good enough when it comes to flying,” Walters said in a university statement. This is true even for the albatross, a specialized long-distance aviator that holds the Guinness world record for his great aeronautical skills.
“What prevents them from being more optimal is probably that they have to land, something that is already difficult for albatrosses but is necessary to reproduce,” he reflected. “A theoretical albatross with an even thinner, longer and more optimal wing may not be able to take off and land safely and therefore would not survive, despite being better formed for flight.”
“The study uses an interesting approach that enriches our knowledge of bird flight.” Michael Pittmana paleobiologist at the Chinese University of Hong Kong told Gizmodo in an email. “There is still much to learn about bird flight and much untapped potential to use this knowledge to deliver bioinspired engineering solutions to society,” added Pittman, who was not involved in the new study.
Overall, the findings demonstrate that a “logical” hypothesis might not necessarily be what nature intended. Many human designs, such as airplane wings and propellers, are modeled after birds, but we may want to be “selective about which animals are inspired,” Walters said.
