Gliding birds tend to have long and thin wings, but seagulls seem to have a trick that lets them soar on short, wider wings
22 November 2022
The dark colours on the top of seagulls’ short, wide wings help change the temperature of the surrounding air, which may allow the birds to glide efficiently without compromising their agility.
Birds tend to have either relatively long, thin wings that support long-haul soaring, or shorter, wider wings that permit more energy-efficient movements like turning and taking off. But some sea gulls have developed wings that can do both, says Madeleine Goumas at the University of Exeter, by absorbing heat from the air.
“Gulls are always flitting about and taking off, and if you’ve got these long wings that are actually adapted for gliding and not lifting your body, it’s going to be quite difficult to get yourself off the ground,” she says. “Once they’re in the air and they’re foraging, then that’s when this dark pigment seems to come into play.”
In 2017, scientists discovered that the black upper surface of albatross wings is about 10°C warmer than the white surface underneath, and that this reduces the density of the air above them. As a result, the viscosity of the air is increased – translating into lowered drag and extra lift for more efficient gliding. After reading about the work in New Scientist, Goumas wondered if there might be a similar phenomenon in sea gulls.
So she investigated the colours, bodies, and wing shapes of 50 species of sea gulls by measuring wingspan compared to wing width – known as the aspect ratio – and body mass compared to wing size – known as wing loading. Then she evaluated how those ratios corresponded with colouring on the back and upper surface of the wings – the mantle – and on the wing tips.
Goumas found that greater wing load was associated with darker shades of grey on the mantle and in the proportion of black markings on the wing tips. More specifically, species with larger bodies and smaller wings, like the great black-backed gull (Larus marinus) and Olrog’s gull (Larus atlanticus), tended to have darker mantles and wing tips, whereas colouring was lighter in birds with smaller bodies and wider wings, like the ivory gull (Pagophila eburnean), which is completely white.
The findings initially appeared to contradict those of another recent study, in which darker wings were associated with longer, thinner wings, Goumas says. But she soon realised that whereas many seabirds like albatrosses – which soar very long distances over the ocean – use both colour and wing shape to maximise their glide, gulls use colour as a “compensation” for their wing and body shapes.
“Gulls are not like albatrosses and lots of other sea birds, because they spend a lot of time on land and are quite sedentary, and they take off a lot to go forage for a while,” she says. “So when I thought about it more, it made sense.”
The findings could eventually inspire engineering designs for more fuel-efficient aircraft, Goumas says.
Journal reference: Communications Biology, DOI: 10.1038/s42003-022-04144-8
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