Bird physiology is conducive to flight: small size, hollow bones, and generally symmetrical feathers on the wings and tail. It seems like a no-brainer that bird evolution was optimized for flight, but a new study suggests that things aren’t as clear-cut as they appear.
The conclusion of a recent study in Nature Communications sounds rather unintuitive: most bird wings don’t appear to be built for maximum flight efficiency and aren’t perfectly fine-tuned for the way birds fly. What’s more, the birds with the most optimized wing shapes in the study were hummingbirds and penguins—with the latter using their wings for proficient swimming, not flying through the air. The researchers analyzed 1,139 images of bird wings with a method that allowed them to test the relative performance of different wing shapes without presumptions on what constituted an “optimal” wing shape.
“For many birds, the functional constraints of flight—the need to generate lift, overcome drag, and turn tightly—do not strongly influence the shape of the wing,” Benton Walters, a doctoral researcher at the University of Bristol in the U.K., 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’s a surprisingly active, heated debate on why some animals evolved to fly. The discourse dates back to the 1880s, but the hypotheses aren’t testable, so it’s a “pointless debate,” according to the late ornithologist John Hutchinson in a blog post for the University of California’s Museum of Paleontology. So instead, Hutchinson added, researchers like to focus on what features enable flight. For modern birds, wings are the “structures underpinning flight,” the paper explained, and birds have “adopted a diverse array of flight styles and high wing shape variability.”
But the more researchers dug into this diversity, the less certain they became about the evolutionary path that led to modern bird wings.
Wings are made…to fly?
Walters and colleagues sought to test the traditional, and arguably intuitive, assumption that bird wings evolved to be best optimized for flight. Simply, natural selection dictates that being better suited 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. “Since flying is a demanding mode of travel and has changed the bird body plan extensively over the 100 million years or so that they have existed, birds should have evolved optimally shaped wings.”
Define ‘optimal’
But it’s typically “very difficult, if not impossible, to determine if an animal is optimal, because there is no way of knowing from looking at living animals alone if the best shape observed is the best overall,” Walters said. To circumvent this challenge, the team chose theoretical morphospace analysis. This method identifies the best evolved shape for a particular flight style, as opposed to assuming that a bird’s current wing shape is the most optimal.
The study analyzed 1,139 images of bird wings and also considered “specialist” flight styles like dynamic soaring and flight styles of species that migrate long distances, Walters said. As expected, flightless birds like ostriches had wings that were “quite unoptimal,” he added.
But the analysis did find bird species whose empirical wing shape closely resembled the predicted optimal wing shape, namely members of the hummingbird and penguin families. Birds with flight styles with high energy demand, such as aerial hawking, also tended to have better optimization. As expected, flightless birds like ostriches had wings that were “quite unoptimal,” Walters explained.
Evolution is complicated
Generally speaking, however, it seemed that for most birds “good enough is good enough when it comes to flight,” Walters said in a university statement. That’s even true for the albatross, a specialized long-distance flier that holds the Guinness World Record for its great aviation skills.
“What prevents them from being more optimal is likely that they have to land, something that albatrosses already have a hard time with but is necessary to breed,” he mused. “A theoretical albatross with an even thinner, longer, more optimal wing may not be able to safely take off and land and thus wouldn’t survive, despite being better shaped for flight.”
“The study uses an interesting approach that enriches our knowledge of flight in birds,” Michael Pittman, a paleobiologist at the Chinese University of Hong Kong, told Gizmodo in an email. “There is still a lot to learn about bird flight and plenty of untapped potential for using this knowledge to deliver bioinspired engineering solutions for society,” added Pittman, who wasn’t involved in the new study.
Overall, the findings demonstrate that a “logical” hypothesis might not necessarily be what nature intended. Many human designs, like airplane wings and propellers, are modeled after birds, but we might want to be “selective about what animals inspiration is drawn from,” Walters said.
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