Hi-res X-ray movies reveal that despite having extremely underdeveloped muscles and wings, young birds may acquire a mature flight stroke early in development by initially relying more on their legs and wings for power, according to a study published April 21, 2016 in the open-access journal PLOS ONE by Ashley Heers from the American Museum of Natural History in New York and colleagues.

This new study may help us understand the development of flight in modern birds, as well as reconstruct the origin of flight in first-winged dinosaurs.

Although scientists have been studying flight for more than a century, there is a surprising amount yet unknown about how birds fly. Unlike adult birds that have large wings and robust, interlocking forelimb skeletons that may help meet the demands of flight, juvenile birds have small "protowings" or "mini wings" and flexible joints that lack many of the hallmarks of advanced flight.

Despite these limitations, young birds can flap their wings as they run up slopes and even briefly fly, challenging longstanding ideas about the origin of flight and flight development.

To gain insight into pre-flight flapping behavior, the authors of this study used X-ray analysis to visualize skeletal movement as developing birds (Chukar partridges, Alectoris chukar) flapped their wings while trying to climb steep slopes.

The authors found that when flap-running at similar levels of effort, juvenile and adult birds showed similar patterns of joint movement. Despite their undeveloped anatomy, young birds appear to produce all of the elements of the avian flight stroke and modify their wing stroke for different behaviors, much like adults.

The authors suggest that the cooperation between a juvenile bird's legs and wings may be key in early life: the force generated by flapping may push the birds forward as well as upward, improving traction as they climb.

While similar observations have not yet been systematically documented in natural settings, the researchers posit leg-wing cooperation may be a bridge between leg- and wing-based modes of locomotion.

The authors also suggest that understanding flapping behavior in young birds may provide insight into the possible use of mini-wings by extinct theropod dinosaurs, 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."

Research paper: Flapping before Flight: High Resolution, Three-Dimensional Skeletal Kinematics of Wings and Legs during Avian Development