Taking Wing
A new view of the origin of bird flight emerges
BOZEMAN, MONT.--It's not often that a presentation given to the Society of Vertebrate Paleontology elicits coos and clucks of sympathy. These are, after all, the scientists who study Tyrannosaurus rex and other fearsome beasts of the past. But that's exactly the reaction Kenneth Dial got when, at the group's annual meeting last October, he showed video footage of a fuzzy little partridge chick with its wings taped to its sides trying to climb a tree--only to tumble down into Dial's waiting hands. Unfettered, however, the chick flapped its tiny wings while climbing and steadily made its way up. After teasing the audience for its sentimental display, the University of Montana biologist returned to the matter at hand: explaining how this and other experiments involving ground-dwelling birds led him to hatch a new hypothesis regarding the origin of avian flight.
Traditionally, scholars have advanced two theories for how bird flight evolved. One of these, dubbed the arboreal model, holds that it developed in a tree-dwelling ancestor that was built for gliding but started flapping to extend its air time. The other, known as the cursorial theory, posits that flight arose in small, bipedal terrestrial theropod dinosaurs that sped along the ground with arms outstretched and leaped into the air while pursuing prey or evading predators. Feathers on their forelimbs enhanced lift, thereby allowing the creatures to take wing.
As the idea that birds descended from dinosaurs gained acceptance by all but a few paleontologists, so too did the cursorial hypothesis. But both the arboreal and the cursorial scenarios have explanatory gaps. As far as tree dwellers go, of the hundreds of nonavian gliding vertebrates around today, not one flaps its appendages. And why would natural selection have favored the development of little protowings in a theropod equipped with heavily muscled legs for running across the ground? Neither theory, Dial asserts, adequately addresses the step-by-step adaptations that led to fully developed flight mechanics.
Dial's eureka moment came after learning that partridges and their fellow ground birds routinely abandon terra firma in favor of trees and other elevated spots for safety. Although these animals appear to fly up into trees, he found on closer inspection that in many cases they were actually running up--legs bent and body pitched toward the tree--while flapping their wings. Subsequent research revealed that wing flapping assists in this vertical running by sticking the bird to the side of the tree, much as a spoiler helps to press a race car to a track.
Although the adult ground birds are generally perfectly capable of flying up trees, their preference for running may stem from a time early in life when they couldn't yet fly: before a baby ground bird has the ability to launch itself into the air, the only means it has for getting off the ground is vertical running. And as Dial's experiments show, when a juvenile is trying to evade a predator this way, the aid of even a partially formed wing can mean the difference between life and death.
Perhaps a bird ancestor's protowing conferred the same benefit, he suggests, and therefore natural selection favored its development. Over time, wings evolved to the point of enabling not only vertical running but, when employed by an animal running across the ground, flight. So far Dial's model has ruffled few feathers. Living animals do not necessarily make good models of extinct ones, however. "Is that the way bird ancestors did it? Well, maybe, maybe not," comments Kevin Padian of the University of California at Berkeley. "But [Dial] is showing that it's possible." For his part, Dial is leaving it to the paleontologists to figure out whether his theory of the genesis of avian flight jibes with future fossil finds--or whether it's for the birds.
