Flapping their tiny wings does not get juvenile partridges off the ground, but it does cast some light on the hidden origins of avian flight.
A new study shows that young partridges flap their wings to help gain a footing as they climb away from predators, although the wings are too small to produce flight. If the ancient ancestors of birds went through a similar stage, it could explain how they eventually evolved wings big enough to fly, says Ken Dial of the University of Montana.
Most paleontologists believe birds evolved from small predatory dinosaurs called theropods. Chinese fossils show long feathers on the forelimbs of some theropods that were clearly unable to fly. Yet how birds, and flight, evolved from these feathered dinosaurs has remained a mystery.
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Competing theories suggest that flight evolved from the ground up, or from the trees down. But neither theory has been able to explain what evolutionary advantage animals might get from an intermediary stage when they sport wings that are too small for flight.
Cheating chicks
To address the question, Dial studied young chukar partridges. The birds nest on the ground, leaving them vulnerable to predators. But the young hatch well-developed and can run up slopes to safer spots from the day they hatch.
Dial observed changes in behaviour as the birds grew, and when he clipped or plucked their wings to change their size. A breakthrough came when Dial鈥檚 teen-age son watched the birds run up the sides of bales of hay.
鈥淢y son said, 鈥榯he birds are cheating鈥� 鈥� by flapping their wings, Dial told 快猫短视频. High-speed photos revealed the running birds beating their wings from head to foot and back. Importantly, the direction of this movement is 90 degrees from that used for flying, which involves flapping from above the back to under the belly.
You can view video of the chick鈥檚 flapping .
Dial found that birds with bigger wings could climb steeper slopes. This suggests that moving the wings up towards the head pushes the birds into the slope, helping their claws gain more traction. More mature birds even used the same flapping style to help scale rough vertical surfaces, like tree trunks.
Birds can climb to heights of five metres to escape predators, but they do it so quickly that the behaviour had been overlooked until now.
鈥業mpeccable鈥� work
Dial鈥檚 observations do not directly address the question of whether flight started on the ground or in the trees.
But if the ancestral dinosaurs could flap their arms in the same way, it does show an evolutionary advantage to evolving small wings 鈥� helping baby dinosaurs climb out of the reach of predators. Evolution of larger wings over time would have led to flight.
Paleontologists generally are impressed by Dial鈥檚 experiments, which Luis Chiappe of the Los Angeles Country Museum of Natural History calls 鈥渋mpeccable鈥�.
However, researchers have no evidence that baby dinosaurs did flap their arms the same way and, without the help of a time machine, they are unlikely ever to do so.
Journal reference: Science (vol 299, p 402)