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Flapping Before Flight

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New research based on high-resolution x-ray movies show that young birds use their underdeveloped muscles and wings to help them move on land, acquiring a mature flight stroke long before they’re ready for takeoff.

Chukar Partridge
Researchers studied the development of flight in young Chukar partridges.
Image courtesy of Wikimedia Commons

The new study, published April 21 in the journal PLOS ONE, is important for understanding the development of flight in modern birds and reconstructing its origins in extinct dinosaurs. 

“The transition from ground-living dinosaurs to flight-capable birds is one of the major evolutionary transitions in vertebrate history, because flight is the most physically demanding form of locomotion,” said lead author Ashley Heers, a postdoctoral researcher in the American Museum of Natural History’s Division of Paleontology and one of numerous Museum scientists whose work is featured in the special exhibition Dinosaurs Among Us, open now. “The kind of flight that we normally think of in living birds involved a huge evolutionary overhaul of the animal’s basic body plan over time. And although scientists have been studying flight for more a century, there’s actually a surprising amount that we don’t know about how birds fly.”

Juvenile birds, like the first winged dinosaurs, lack many hallmarks of advanced flight. Instead of large wings, they have small “protowings,” and instead of robust, interlocking forelimb skeletons, their limbs are slimmer and their joints less constrained. These same traits are often thought to preclude extinct theropods—the group of dinosaurs most closely related to modern birds—from powered flight.

An illustration showing what Microraptor may have looked like in life.
Zhao Chuang; courtesy of Peking Natural Science Organization

To further explore this idea, Heers and her colleagues used a technique called x-ray reconstruction of moving morphology (XROMM)—which essentially produces a 3D x-ray movie—to visualize skeletal movement in developing birds. At Brown University, the researchers used XROMM to look at Chukar partridges (Alectoris chukar) at a variety of ages as they flapped their wings to help climb steep slopes—a behavior scientists call wing-assisted incline running (WAIR).

They found that when flap-running at similar levels of effort, juvenile and adult birds show similar patterns of joint movement. Despite their undeveloped anatomy, young birds can produce all of the elements of the avian flight stroke and modify their wing stroke for different behaviors. This wing-leg cooperation is a bridge between leg- and wing-based modes of locomotion, and the study indicates that extinct theropod dinosaurs might have done the same thing with their mini-wings before flight evolved.

“Baby birds anatomically look a lot like some of the dinosaur fossils that we see,” Heers said. “And so, by studying baby birds and looking at how they actually use these dinosaur-like anatomies, we can get a better sense of how these long-extinct animals might have been using their wings.”