By Natalie Parletta
The way birds move their wings, rather than the shape of those wings, determines how they fly, new research shows.
“A bird in flight does not strictly rely on having its wings fully extended and held in a stiff posture,” explains Vikram Baliga, from Canada’s University of British Columbia.
“Rather, birds fold, unfold, bend, and twist their wings as part of normal flapping behaviour.”
A comparison, he says, is how humans use their arms and adjust their stroke when they swim. Birds move through the air in a way that causes dynamic changes to wing shape known as “wing morphing”.
This seems to be common for most birds and is particularly evident when they react to sudden gusts of wind or escape predators.
Baliga and his colleagues wanted to determine how the capability to morph the wing relates to the variety of behaviours a bird may show – but flight behaviour is tricky to measure.
Their approach, described in the journal Science Advances, was to look for common patterns of wing use – like flapping, soaring, gliding and hovering – among different bird species.
Identifying 12 potential wing-use behaviours, they ran an algorithm and were able to group 61 species together by strongly shared patterns of flight style.
They then examined various “range of motion” variables along with standard measures like wing shape using cadavers from each species sourced from UBC Beaty Biodiversity Museum.
From cadavers identified as fresh and untainted by rigor mortis, they manually moved the wings through their range of motion, recording their capacity to fold, unfold, bend and twist by placing markers at key points.
The key finding is that a wing’s range of motion corresponds more strongly to a bird’s size and how it flies than to the wing’s shape.
The flying capabilities derive from the underlying skeleton; “it’s all in the shapes of the bones and how the bones interact,” Baliga says. “We found that nature has consistently reshaped the skeleton so that the range of motion evolves in concert with the evolution of flight and body size.”
It was somewhat surprising, he says, as he expected the range of motion would be similar across species and that flight would vary according to the part of that range each bird used by contracting specific muscles in their wings.
The wings of species that flap and bound, like finches (Fringillidae) and thrushes (Turdidae) for instance, have joints that allow a lot of freedom of movement.
In contrast, birds like eagles (Accipitridae) that glide and soar have a very limited range of motion.
“The shapes of the bones in a bald eagle’s wrist help restrict its wing’s freedom of movement,” says Baliga, “which could be important to keeping steady posture during gliding and soaring.”
Species like the murre (Uria aalge), that swim and propel themselves underwater, have even stronger restrictions to bending and twisting their wings, which might help the birds use them like a paddle.
Baliga says these insights into the importance of size and flight strategy could help inform the design of aircraft and drones.