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Re: four winged Archaeopteryx

On Sat, Sep 30, 2006 at 06:09:47PM +1000, Dann Pigdon scripsit:
> Graydon wrote:
> > Feathers started as insulative structures, not aerodynamic structures.
> > 
> > All known birds can fluff their insulative feathers, and it's very
> > hard to see how a non-mobile insulative structure could survive
> > selection pressure, so the least hypothesis is that the proto-feather
> > could move.  Not a lot, but there would have to be some mechanism to
> > raise and lower it to control the amount of trapped air.
> Feathers are only raised to help a bird to cool down. 

When the small birds around here are doing the tribble-with-a-beak
immitation, it's because it's cold out, not because it's hot out.

Raising feathers increase the amount of trapped air and increase the
amount of insulation.  While I can see this being useful to avoid
picking up heat in a brief transit from on ecool place to another --
going from deep shade to deep shade across tropical sunlight, say -- I'd
expect that the primary use is to prevent heat loss, the same as with
raised fur in mamals.  

Raising feathers to the point where there's no trapped air I've only
seen during preening, and I've no idea how much of that was done with
the beak.  Since birds don't sweat I'm not sure it would be a net win
for cooling even if sunburn wasn't a concern.

> I doubt that a long thin appendage like a bony tail - with likely
> little flesh and a huge surface area to volume ratio - would
> specifically need cooling down (in fact, the opposite is more likely).
> Therefore tail feathers in early volant theropods may never have had
> any insulatory mobility to begin with.

Whereas I'd consider a long thin bony tail to be a primary candidate
for insulative feather mobility, just to keep it from freezing off.