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] Please Register or Log in to view the hidden image! 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): Please Register or Log in to view the hidden image! 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. Please Register or Log in to view the hidden image! 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.
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.
I avoid taking off uphill whenever possible, because it's often scary, and the experience usually involves wishing I could beat my wings.
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.
