Birds come in an astounding array of shapes and colors, but it’s their physical prowess – like a bald eagle’s incredible ability to soar that captivates human imagination.
An enduring mystery is why bird species with similar flight styles or body sizes don’t have consistent wing shapes.
How Nature Reshapes Wings
All hummingbirds and some species of falcons, hawks, kingfishers, and passerines can hover.
But the birds have strikingly different morphologies and are only distantly related.
Ravens soar like eagles, while their look-alike cousins, crows, stick more closely to the ground.
Why Bald Eagles Soar
New research in Science Advances offers an explanation.
Bird species tend to reshape the range of motion of their wings—rather than wing shape or size itself—as they evolve new ways of flying.
Birds Swim Through the Air
“Birds essentially swim through the air—they flex, extend and bend their wings in flight,” explains Vikram Baliga, a researcher at the University of British Columbia.
“As a bird specializes in a flight style, nature doesn’t appear to reshape the size or shape of the wing as much as it remodels the wing’s range of motion. Much like a swimmer adjusting their stroke.”
According to the research, hovering birds are relatively restricted in their ability to extend their elbows, but have a generous capability to move their wrist.
“Hummingbirds basically tuck their elbows in and predominately rely on rapidly swinging their wings at the wrist joint,” says Baliga.
Soar and Glide
“For birds that glide, it’s more about maintaining the position of the limbs to keep steady sail,” he says.
Baliga says the most restricted species in the study are the bald eagle (Haliaeetus leucocephalus) and the sooty shearwater (Ardenna grisea).
These are two birds that frequently soar and glide.
Measuring Bird Species’ Wings
Baliga and UBC zoologists categorized 61 species of birds by flight style from hovering to gliding to soaring.
Using samples collected by the UBC Beaty Biodiversity Museum, the researchers manually measure the shape, flexibility, and extendibility of each species’ wing.
Then they built an evolutionary family tree of the birds to determine how the range of motion evolved in the wrists and elbows of bird’s wings.
Engineers Look to Birds to Improve Flying Performance
This work provides insights for drone and aircraft design as engineers are looking to nature to improve flying performance.
“By looking across avian flight diversity, our research has determined one component of how birds use their wings,” says Baliga.
He says researchers are working towards understanding how wings in nature morph during flight.
This knowledge can be applied to unmanned aerial vehicles—particularly in turbulence, wind gusts, or when attacked by aerial predators.
Evolution tests a range of wing designs and motions for specific circumstances.
Looking at the restrictions that nature places on birds of different sizes and flight styles can help.