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 These are the problems I'm having with the ground up hypothesis; assuming that 
protobirds already had elongated feathers on their arms, what advantage would 
this give a ground running predator? If they leapt into the air, these "wings" 
would actually slow them down due to drag, making them more vulnerable to 
predators. I think a more streamlined animal with powerful legs and no "wings" 
at all would make more sense for a specialized "jumper" (kangaroo). Also, why 
would they be leaping up into the air in the first place? To catch insects? No 
animal that does this exists. If they evolved simple wings to provide lift 
while running, wouldn't this actually reduce traction and in turn slow them 
down and make them less stable? To me, an at least somewhat arboreal animal 
that used it's elongated feathers or even protofeathers to parachute from an 
elevated position makes more sense; I will not recite all the proposed stages 
of flight that would gradually develop from that that point, because
 others have already explained it better than I ever could.

  Regarding potential arboreality in archaeopterx; I think one only needs to 
look at the modern hoatzin chick 
http://en.wikipedia.org/wiki/Image:Hoatzin_chick.jpg. Here is a fully modern 
bird that, in the early stages of its life, has claws that are very similar to 
those found on archaeopterx, which it uses purely for climbing. Now I know this 
is probably the 10,000th time this has been brought up, but I think it's a 
pretty damn good example of a modern flightless dinsoaur possibly reverting 
back to its ancestral condition. Some claim archaeopteryx lacks adaptations for 
arboreality, yet they had greatly elongated fingers that would have been good 
for little else other than climbing, or swimming. Primitive bats had greatly 
elongated fingers with claws on all five digits which they used for climbing as 
well. Fossil bats make a good case for at least one group of flying animals 
evolving from arboreal gliders. 

  There is a strong selective advantage for an arboreal animal to evolve some 
sort of parachute or gliding system to prevent injury from falling or to 
conserve energy by leaping from tree to tree instead climbing all the way down 
and back up a tree trunk, or cliff face: this is made obvious by the large 
number of arboreal creatures that have evolved in this way. On the other hand, 
you do not see any primarily ground running, leaping animals evolving some sort 
of simple gliding or flight system; the only animals that do anything like this 
are birds, yet they already have a fully developed and highly sophisticated 
flight mechanism. There are plenty of jumping, kangaroo like animals, yet none 
of them have evolved wing membranes or any organ other than stronger legs (and 
smaller arms) to increase airtime.

 I know my earlier statements were pretty unscientific (if not downright 
ignorant) but at least it got some people riled up and resulted in some very 
informative, thought provoking responses.

I love this Mailing list =)