Be it finches flapping, hummingbirds hovering or seabirds soaring, birds are capable of achieving all manner of aerobatic approaches to flight. It would not be unreasonable to assume that these specialised flight behaviours would be associated with specific wing types, but, in fact, many birds with similar flight styles possess wings of varying shapes and sizes. One theory for this inconsistency between form and function is that traditional 2D wing shape measurements are a poor representation of the way birds actually use their wings during flight. By focusing instead on the wing's 3D movements, or their ‘range of motion’, a team of researchers from the University of British Columbia, Canada, led by Doug Altshuler recently revealed that when it comes bird flight behaviour, it's less about the wings you've got and more about how you use them.
To clear up the conflicting relationship between wing morphology and flight behaviour, the team started by acquiring examples of 61 bird species and measured the area, shape and the ratio between length and width (known as aspect ratio) of the extended wing. Next, the team identified each wing's full range of motion by marking locations on the wing, such as the elbow and wrist joints, before manually flexing and extending the wings – while filming the motion of the markers from multiple angles to reconstruct the motion in 3D. In order to compare these hand-made range of movement measurements with the animal's free movements, the team also filmed two of the 61 species, pigeons (Columba livia) and zebra finches (Taeniopygia guttata), flying from various angles. After researching in the literature what was already known about bird flight, the team then assigned each of the 61 species to at least one of 12 distinct categories of flight styles, including different combinations of hovering, gliding, soaring and flapping flight. Finally, to investigate how wing shape and flight behaviour varied between related species, the team constructed a family tree from the DNA sequences of 220 bird species and paired each species with their flight styles.
The team's experiments confirmed the theory that the flight style of a bird is much more strongly linked to the wing's range of movement than its static shape, in addition to finding that the wings’ range of movements were almost twice as likely to correctly predict a bird's flight behaviour than their wing shape or body mass. These results reveal that bounding and gliding birds tend to have a greater range of wing movement and lower body mass, allowing for a wider adaptability of wing motion, while soaring birds, such as eagles, tend to possess rigid wings with a much more restricted range of movement. The team also report that pigeons and zebra finches rarely fully extend their wings when flying freely, further suggesting that wing shape does not accurately represent a bird's true range of flight styles.
To explain these findings, the team then turned to their evolutionary tree and realised that 2D wing shape is much more similar between related species than the wing's range of movement, suggesting that bird species are more likely to vary their flight behaviours through 3D wing motion than by adapting the shape of their wing. Not only do these results help to improve our understanding of bird flight behaviours and the evolutionary processes behind them, they also show promising applications for our own aeronautical ambitions. For drones and crewed aircraft, overcoming turbulence and strong winds are issues that could be addressed by morphable wing shapes, and maybe one day soon, this area of research will influence the shape of wings to come.