Flapping a forerunner to flight?
Ancestral bird might have moved like chicken.
17 January 2003
KENDALL POWELL
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Partridges flap to increase traction. © Science / Illust.
by R. Petty
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Chickens have always seemed like farmyard failures at flight. But their
flap-and-run technique may represent just the leap needed for theories on the
origin of flight to get off the ground.
Lab treadmill tests show that partridges and other ground-based birds
such as chickens, turkeys, quail and grouse flap their small wings while
running to improve traction on steep inclines and vertical surfaces1.
The finding, by Kenneth Dial of the University of Montana in Missoula,
provides a new model for how flying dinosaurs and ancestors to birds may have
evolved.
"Their wings are being used not to take them to the heavens - that's
where our heads have been - but to stick them to the ground," Dial says. A
modified flap-running may have helped the first flyers climb trees or cliffs to
take refuge from predators and give them a more controlled descent back down,
he proposes.
Fully grown partridges can reach vertical heights of more than five
metres without a running start, and can even navigate overhangs 10 degrees
beyond vertical. Baby birds with immature wings perform almost as well as
adults.
The birds cannot perform their wall-clinging trick as well when Dial
trims or plucks their wing feathers. And all birds, fully feathered or not,
have a hard time climbing a smooth surface.
By placing accelerometers on the birds' backs and taking high-speed
videos, Dial determined that the birds' wings produce a force that pushes the
animals towards an inclined surface. In the video, he saw birds swivel their
wing position to match the steepness of an incline.
In other words, the flapping wings act like the spoiler on the back of a
racing car, which aerodynamically pushes the car towards the track for better
traction. Dial proposes that this new-found function for wings may mimic an
intermediate step in the evolution of flight that biologists had
overlooked.
No one can assume that Dial is seeing an ancestral state in his birds,
says palaeobiologist Kevin Padian of the University of California, Berkeley,
but the work shows an ability that ancestral birds may have used. "Until you
have a fully functional wing, you can't fly," he says. "But a precursor wing
would give an aerodynamic advantage that could help you run up a tree
trunk."
Winging it
Many biologists believe that birds sprung from a type of dinosaur known
as theropods. These had similarities with ground birds: they had strong
hindlimbs, they ran quickly along the ground, and some had feathered forelimbs.
The winged dinosaur Archaeopteryx had a shoulder joint that allowed its wings
to swivel in several directions, instead of just flapping perpendicular to the
body as in flight.
The question of whether flight evolved from the ground up, from trees
down, or from some combination of the two is an ongoing debate. Richard Prum,
ornithologist at the University of Kansas Natural History Museum in Lawrence,
says: "The main question now is which components of avian flight evolved in the
context of terrestrial locomotion and which evolved in the context of aerial
flight?"
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