The Evolution of Flight

[S.A.M.]

A study published in the journal Nature suggests that the key to understanding the evolution of bird flight is the angle at which a bird flaps its wings.

The US team found that birds move their wings at the same narrow angle, whether they run, fly or glide.

They conclude that early birds may have begun to fly by simply learning to flutter their wings at the right angle.

Scientists investigating this area tend to fall into two camps, he said. Those who believe that birds learned to fly from the "top down" - by falling out of trees and gliding, and those who think that birds took to the air from the "ground up" - by running and flapping their wings, possibly to escape predators.

However, both of these scenarios suggest that birds would first need to establish a wide range of wing movement in order to become airborne.

http://news.bbc.co.uk/2/hi/science/nature/7205086.stm

Here is the abstract of teh original paper:

A fundamental avian wing-stroke provides a new perspective on the evolution of flight.
Dial KP, Jackson BE, Segre P.
Nature. 2008 Jan 23 [Epub ahead of print]

The evolution of avian flight remains one of biology's major controversies, with a long history of functional interpretations of fossil forms given as evidence for either an arboreal or cursorial origin of flight. Despite repeated emphasis on the 'wing-stroke' as a necessary avenue of investigation for addressing the evolution of flight, no empirical data exist on wing-stroke dynamics in an experimental evolutionary context. Here we present the first comparison of wing-stroke kinematics of the primary locomotor modes (descending flight and incline flap-running) that lead to level-flapping flight in juvenile ground birds throughout development (Fig. 1). We offer results that are contrary both to popular perception and inferences from other studies. Starting shortly after hatching and continuing through adulthood, ground birds use a wing-stroke confined to a narrow range of less than 20 degrees , when referenced to gravity, that directs aerodynamic forces about 40 degrees above horizontal, permitting a 180 degrees range in the direction of travel. Based on our results, we put forth an ontogenetic-transitional wing hypothesis that posits that the incremental adaptive stages leading to the evolution of avian flight correspond behaviourally and morphologically to transitional stages observed in ontogenetic forms.

Fig 1. The fundamental wing-stroke described herein is used days after hatching and during all ages and over multiple behaviours (that is, flap-running, descending and level flight) and is the foundation of our new ontogenetic-transitional wing hypothesis. At hatching, chicks can ascend inclines as steep as 60° by crawling on all four limbs. From day 8 through adulthood, birds use a consistently orientated stroke-plane angle over all substrate inclines during wing-assisted incline running (red arcs) as well as during descending and level flight (blue arcs). Estimated force orientations from this conserved wing-stroke are limited to a narrow wedge



This reminded me of this paper I read some years ago (by the same group):

Adult birds fully capable of aerial flight preferentially employ wing-assisted incline running (WAIR), rather than flying, to reach elevated refuges (such as cliffs, trees, and boulders). From the day of hatching and before attaining sustained aerial flight, developing ground birds use WAIR to enhance their locomotor performance through improved foot traction, ultimately permitting vertical running. WAIR provides insight from behaviors observable in living birds into the possible role of incipient wings in feathered theropod dinosaurs and offers a previously unstudied explanation for the evolution of avian flight.

Fig 2. (A) Incline-running performance on a textured substrate (36-grit sandpaper) for chukar partridges with fully feathered (control group) wings during development from posthatchling to 50 days. Shaded area represents angles of shallow incline where birds did not recruit their flapping wings. To ascend steep inclines, developing chicks and adults employ WAIR (nonshaded area). (B) Incline-running performance on textured and nontextured (smooth) substrate for chukar partridges possessing fully feathered (control, C), trimmed (T), and plucked (P) wings starting the day after hatching. Data points represent the climbing angle (in 5° increments) that all five individuals within each of the three groups were able to perform that day. Control animals (feathered wings) were capable of vertical running within 20 days of hatching, whereas plucked birds did not improve incline running performance beyond what they could attain during their first few days posthatching. Birds with trimmed wings and incapable of aerial flight attained intermediate locomotor performance. These data show that hindlimb traction is associated with WAIR performance.

Fig 3. From left to right, chukars are shown running at inclinations of 0°, 60°, 70°, 80° and 90°. The arrows represent the direction and magnitude of the maximum ground reaction force incurred by the hindlimbs pushing against the substrate. The force remains directed away from the substrate (the birds are pushing down to support the body), but on slopes the force increases and tilts forward (the birds are accelerating vertically along the incline). Even at the 90° inclination, added traction from the wingbeats allows the hindlimbs to exert large forces against the substrate, supporting the body and propelling it up the slope.

 

[S.A.M.]

Can everyone see the pictures? I have access so I cannot tell the difference.

 

[iceaura]

I can, with no privileges.

The discovery that the wingstroke greatly improved foot traction and leg power during near vertical climbing, without flying, was new to me - an obvious line of development, once noticed. Two legged animals have a hard time climbing at speed.

 

[hypewaders]

I avoid taking off uphill whenever possible, because it's often scary, and the experience usually involves wishing I could beat my wings.

 

[Fraggle Rocker]

The videos I've seen show birds that can't fly using their wings to create negative lift when climbing a steep surface, providing more traction for their feet. They can even go beyond 90 degrees. This is a tremendous evolutionary advantage in predator evasion.

We may never know, but it's been suggested that wings might have served this purpose first, and as they grew stronger and the feathers grew larger, they eventually became capable of creating significant positive lift. An intermediate stage of gliding, like flying squirrels, would not be remarkable.