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Feather colour might make a difference to flight efficiencyHarry Collins / Alamy
By Donna LuDark feathers may help birds fly more efficiently. They heat up the animals’ wings and the surrounding air, which might help increase airflow over the wing.
Svana Rogalla and her colleagues at the University of Ghent studied several bird species to see how the colour of feathers affects wing temperature during flight. They thermally imaged an osprey and found that dark feathers on the bird become warmer than light ones.
To see how wing temperatures changed during flight, the team then used stuffed bird wings of different species in a wind tunnel, and heated them with infrared light bulbs similar in intensity to being outdoors on sunny and cloudy days.
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Soaring birds
They exposed osprey, gannet and back-blacked gull wings to wind speeds of 6, 12 and 18 metres per second, similar to the birds’ natural flight speeds. “We wanted to simulate flight under realistic conditions,” says Rogalla.
The species chosen were large soaring birds, which can increase their altitude without flapping their wings by riding on rising air currents.
The team heated each wing for two minutes before placing it in the wind for two minutes.
Darker feathers heated up much more than light feathers. “We found temperature differences of about nine degrees between black and white,” says Rogalla. “We would even find these temperature differences in the same wing.”
Wing patterns
The research suggests that a common wing pattern – white feathers at the root of the wing where it attaches to the body and black feathers at the wing tips – could increase lift while flying. The temperature difference between the light and dark feathers creates convective currents in the air above the wing that move from the cooler base of the wing to the darker tips. This boosts the airflow over the wing, which may make flight more efficient.
Several bird species including storks, gulls and pelicans have white wing roots and black wing tips, although there is variation between individuals. “It could help to have these two different colours in one wing to improve flight,” says Rogalla.
The next step is to study the precise effects of different temperatures on lift and drag during flight.
Journal reference: Journal of the Royal Society Interface, DOI: 10.1098/rsif.2019.0032
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