Hummingbirds' wings 'shape-shift'
Footage shot with high-speed cameras has revealed how hummingbird wings bend and flex, to keep the birds in the air.
Masateru Maeda, a PhD student at Chiba University in Japan, captured the footage.
The ultimate aim of his measurements of the movements of the wings is to copy their function in the design of flying robots.
The scientist presented his findings at the Society for Experimental Biology's annual meeting in Valencia, Spain.
The researchers captured their footage at Tama Zoological park in Tokyo.
Flapping and flying
As birds and insects move through the air, their wings are held at a slight angle, which deﬂects the air downward.
This deflection means the air flows faster over the wing than underneath, causing air pressure to build up beneath the wings, while the pressure above the wings is reduced. It is this diﬀerence in pressure that produces lift.
Flapping creates an additional forward and upward force known as thrust, which counteracts the insect's weight and the "drag" of air resistance.
The downstroke or the flap is also called the "power stroke", as it provides the majority of the thrust. During this, the wing is angled downwards even more steeply.
You can imagine this stroke as a very brief downward dive through the air - it momentarily uses the animal's own weight in order to move forwards. But because the wings continue to generate lift, the creature remains airborne.
In each upstroke, the wing is slightly folded inwards to reduce resistance.
The team chose hummingbirds as their "wing model" because they can be studied so easily; they hover quite still as they feed on nectar.
"And they're very small," added Mr Maeda. "Larger birds that cannot hover have to be studied in wind tunnels."
But to get his footage, Mr Maeda had to work in the glasshouse of the zoo, which is kept at 35C for the birds and butterflies that live there.
He trained his high-speed camera on a nectar feeder in order to capture shots of hummingbirds as they hovered.
"We are curious about the precise wing shape," Mr Maeda told BBC News. "The feathers [move and] change the wing area as they are flapping."
This movement of the primary flight feathers, the researchers found, changes the shape and size of the wing in such a way as to very precisely control the lift they generate.Flapping robots
The eventual aim is to design wings for miniature flying robots that would be able to perform in a similar way.
Mr Maeda and his colleagues have already started a collaboration with engineers at Chiba University to design a flexible, bird-inspired wing.
The team also hopes to learn more about the hummingbirds' impressive hovering capabilities.
Mr Maeda said that the birds must have a very acute sense of their wings' shape in order to remain so still in the air.
"If the wing shape isn't optimised," he explained, "it will fail to produce lift and the bird will start to sink.
"So it must be able to sense this and correct the shape of its wings."