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Re: Fw: Dinosaurs and birds
On Wednesday, April 4, 2007, at 08:08 PM, don ohmes wrote:
Not sure I understand, from the perspective of a 'ground-up' selective
process that can transform a terrestrial mud-lover into a barn
swallow, where the line between volancy and various forms of
wing-assisted running is (inclines are NOT necessary, in my opinion).
Once Archies ability to flap synchronously is established, then the
question of volancy in Arch. requires drawing that line, does it not?
So my question is, has that been done?
The incline is necessary, because without it there is no requirement to
produce a lift force towards the substrate, which is the critical
aspect of wing assisted running. In the WAIR dynamics, the flapping is
not utilized to generate better climbing speed via thrust; the bird is
generating increased vertical momentum indirectly by elevating the
frictional force on the hind limbs. If the animal is running on a
level substrate, then producing a lift force is usually not
particularly helpful, and can actually be counter-productive under some
scenarios (as would probably be the case for basal birds).
Many living birds utilize running takeoffs, and some of them flap
heavily while launching. However, most of those birds are semiaquatic,
and the wings are important in the early to mid launch mostly because
the lift elevates the bird slightly from the aquatic medium and allows
for the animal to run on the water surface to reach its launch window.
Only late in the running launch is the oscillation of the wings really
helping the bird get into the air directly. Most birds use a leaping
launch. In that case, the wings contribute more heavily to reaching
flight speed, and are brought to bear in full earlier in the launch
cycle. Also note that a running launch is derived in semiaquatic
birds, and not the basal condition.