IT’S not every day that an aerospace engineer raises new questions about bird flight. But Abdessattar Abdelkefi and his team at New Mexico State University did just that while trying to devise better drones.
Many large soaring birds like the albatross have wings that are white underneath and black on top. Previous explanations focused on camouflage, says Graham Martin at the University of Birmingham, UK.
But does that colouring really boost endurance in flight? Most soaring needs no flapping of wings; instead, the bird exploits air currents to glide.
Abdelkefi’s team discovered that a wing’s black upper surface absorbs sunlight very efficiently, causing it to be around 10°C warmer than the lower surface. That effectively lowers air pressure on the upper surface, lowering drag and generating extra lift (Journal of Thermal Biology, doi.org/f96ggw).
Svana Rogalla at the University of Ghent, Belgium, says thermography has proved that the dark upper wing gets hotter in sunlight, but it is too early to pin down its effect on drag. The impact of colour on flight could be a further inducement for birds to make costly melanin pigment to darken feathers, she says.
The team hopes the findings will help them design more efficient and durable drones for use at sea.