In a new study published Thursday to the journal Science, a team of Chinese researchers describes the presence of feathered, wing-like hind legs in primitive birds. According to the researchers, these findings confirm that many species of early birds used four wings to fly. The team gleaned the results by examining eleven fossilized specimens at Tianyu Museum of Nature in China's Shandong province.
Scientists actually don't have to visit a museum to witness birds with feathered legs, they just have to look at many species of vultures and eagles. However, these birds' leg feathers are thinner and wispier, primarily functioning as insulation. The feathers found on the hind limbs of the fossilized early bird specimens all had curved shafts, were perpendicular to the long axis of the leg, and were arranged in a fan-like manner. Clearly, these were meant for flight.
Archaeopteryx, perhaps the most well known prehistoric bird, is also thought to have sported curved "flight" feathers on its hind limbs. However, they weren't arrayed like a wing would be, making the appendages more leg-like than wing-like. Thus, the legs were likely supplemental, not essential to flight.
The current novel research is the first confirmation that four-winged flying made the evolutionary transition from bird-like dinosaurs to early birds. The second pair of wings likely provided lift and enhanced maneuverability. According to Drs. Michael Habib and Justin Hall of the University of Southern California, hind wings allowed the bird-like dinosaur Microraptor (pictured above) to reduce its turning radius by 40% and almost triple the speed of a turn!
This begs the question: with such overt benefits of four wings, how did less become preferable to more?
While hind wings boosted maneuverability they also likely took at toll on early birds' aerodynamics -- increasing drag and reducing top speed. Switching to a single pair of wings and replacing hind limb feathers with scales made flight far more efficient. Moreover, it allowed birds to excel at ground locomotion in addition to aerial locomotion.
That's why -- as far as wings go -- two is better than four, and less is more.
Source: Xiaoting Zheng, Zhonghe Zhou, Xiaoli Wang, Fucheng Zhang, Xiaomei Zhang, Yan Wang, Guangjin Wei, Shuo Wang, Xing Xu (2013) Hind Wings in Basal Birds and the Evolution of Leg Feathers. Science Vol. 339, DOI: 10.1126/science.1228753
(Image: Microraptor via Shutterstock)