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Origin of bird flight: ontogenetic-transitional wing (OTW) hypothesis

Kenneth P. Dial, Brandon E. Jackson & Paolo Segre (2008).  A fundamental avian 
wing-stroke provides a new perspective on the evolution of flight.

Nature advance online publication 23 January 2008 | doi:10.1038/nature06517; 
Received 20 August 2007; Accepted 27 November 2007; Published online 23 January 

"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°, when referenced 
to gravity, that directs aerodynamic forces about 40° above horizontal,
 permitting a 180° 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."

Later in the paper appears this brilliant statement, which hits the nail right 
on the head...

"Perhaps we can cut the Gordian knot created by the false dichotomy of the 
highly charged, but unresolved, cursorial–arboreal debate.  The OTW hypothesis 
embraces salient features of both the arboreal and cursorial hypotheses yet 
clearly differs from both."

Although I don't necessarily agree with everything in the paper (only ~ 
90-95%).  For example, I'm a bit skeptical about this...

"Commonly held assumptions within the cursorial school about the plausible 
function of proto-wings are inconsistent with the ontogenetic biology of extant 
forms; for example, no extant species uses its wings to run faster, to secure 
prey or run–glide."

No extant avian uses its forelimbs to catch prey, because these forms already 
have the forelimbs highly modified into wings; or else they are flightless 
birds that evolved from forms that had the forelimbs highly modified into wings.



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