The path to flight takes legwork, say scientists

A new study has found a link between legs and wings in developing birds. The link could provide clues as to how dinosaurs evolved into flying birds.

How did earthbound dinosaurs become today's soaring birds? The evolutionary path to flight has baffled scientists in the past, but teamwork between young nestlings' legs and arms may provide a clearer picture of how their taxing trademark movement developed, according to a new study. 

A team led by Ashley Heers, a postdoctoral fellow at the American Museum of National History, took a closer look at how baby birds use their wings before they are flight-ready. The new study, published Thursday in the journal PLOS ONE, found the wings worked in conjunction with the animals' legs to unlock a range of locomotion not previously thought possible – a link that could also help explain how dinosaurs transitioned from the ground to the air. 

"Transitional behaviors are a big puzzle," Ashley Heers said in a phone interview with The Christian Science Monitor. "We know they involved dinosaurs that can't fly and then end with animals that can fly, but what’s going on in between?"

Part of the problem in finding the link may have been too much attention on the final stage.

"People have been studying flight for over 100 years, but everyone has focused on adult birds" who fly, Dr. Heers says.  But it's the baby birds, with underdeveloped muscles, wings, and feathers, that most resemble modern birds' ancient dino ancestors, the theropods. 

Adult birds have a host of anatomical features that are designed to aid flight. Long wings a relatively restricted joints appeared to make flight possible. The absence of those elements in young birds and theropods were thought to be what kept them back from flight.

But despite lacking the necessary anatomy, young birds often flap their "protowings" very similarly to adult birds. "If you look at their skeletons ... they shouldn’t be able to do that," Heers tells the Monitor.

To get a better view of what was happening behind the scenes, her team recorded X-ray movies of the young birds using their wings to power up inclines – a process scientists refer to as wing-assisted incline running (WAIR).

Footage of the birds was gathered using X-ray reconstruction of moving morphology, a new X-ray technique that involves two cameras recording from different angles, allowing researchers to view movements in three dimensions.

The scientists observed various sets of birds by age, starting with a group one week old. The findings showed that the little ones were able to produce mature flight strokes with their wings, even without adults' anatomical advantages. Birds 18-20 days old were even capable of flight, although they still lacked many of the grownups' features.

"We've assumed for a really long time that in order for a bird to fly they need this suite of aerodynamic factors, but now we're seeing that's not necessarily true," Heers explains.

The missing link in other research, she says, was the relationship between young birds' wings and legs.  

At first young birds rely heavily on their legs and use their wings to produce extra force when going up inclines. As they get older, wings are able to carry more of the weight, literally. This cooperative link between wings and legs allows modern birds to "seamlessly transition from an animal that can't fly to an animal that can," Heers says. And it's a link that may have helped dinosaurs evolve on the path to modern flight, as well.