Re: Archaeopteryx with bird book, was Re: Archaeopteryx flight

[Tim Williams <twilliams_alpha@hotmail.com>]

Patrick Norton wrote:

> Where is the evidence of selection for baldness in scavenging birds?  Other 
> than vultures, I can't think of any other type of scavenging bird that is 
> bald (crows, ravens, magpies, gulls as well as some types of owls, herons 
> and raptors). And several of these types of scavenging birds are not much 
> bigger than Archie.

I can answer that question in more than one way.  Firstly, there are two 
separate families of vultures (Old World and New World) that evolved 
independently.  There is also the marabou stork, a scavenging bird that also 
has a bald head.  So, that's three separate examples of baldness associated 
with scavenging in large birds.

Secondly, the life-style of scavengers like vultures is very specialized.  
They will plunge their neck and heads deep into the carcass, and some 
species specialize on feeding on the viscera (accessed via the anus).  The 
naked head and neck of vultures and the marabou is an adaptation to feeding 
on (or rather, *within*) large mammal carcasses, from which they will gulp 
down large chunks of flesh or viscera.  Thus, these birds will actually 
immerse the head and neck inside the rotting carcass.  The other birds you 
mention (crows, ravens, gulls, etc) tend to be smaller species that peck at 
dead prey, and are usually more fastidious about the choice of prey and how 
deep they will go inside the carcass.

> The absence of head feathers in Archie is far more likely an artifact of 
> preservation than evidence of baldness.

You're probably correct.

Jerzy Dyczkowski wrote:

> In total, maybe 1% of living birds have bald heads. Scavenging birds are 
> much bigger than A., possibly because carrion is scarce resource and they 
> need passive flight to cover vast areas to locate it.

Yes.  I am in no way suggesting that _Archaeopteryx_ was an obligate 
scavenger.

> Agreed. Common assumption is that center of mass of A. was further back 
> than of modern birds because of bony tail. However, front of A. was also 
> more heavy, including toothed jaws. So it might not be true.

Also, many Mesozoic birds retained teeth in the jaws, including flying 
species (e.g., many enantiornithines).  So I wonder just what impact (if 
any) the presence teeth had on the center of mass.

> For CM in A. most important is size of sternum and breast muscles and this 
> is unknown and debated. So my idea that examination of leg joints is needed 
> to decide it.

Yes, including the robustness on the femoral shaft.  I suspect that 
_Archaeopteryx_'s locomotory posture on the ground was closer to that of 
non-avian theropods than to modern birds, and that the femur continued to 
play a major role in stride generation.

> So I still maintain that A. was frequent walker but not runner - more like 
> a pigeon than roadrunner.

This is interesting.  I wonder if this means that _A._ was incapable of a 
running take-off...?

James Cunningham wrote:

> There is some circumstantial evidence against it, in that Archaeopteryx is 
> known to have a rather substantial tail, that acts to move the cg 
> aftwardly, and is known to have tail feathers on that tail, capable of 
> supporting the weight of the tail and hindlimbs in flight.

You've raised this "third-wing" idea before on this list, and it rings true 
to me.  (Not that I fully understand the aerodynamics; this isn't really my 
field.  But it certainly explains a lot.)  I would suggest that this 
"third-wing" (rectricial 'frond') is a carryover from the 
pre-_Archaeopteryx_ stage, where it served the same function in gliding.

Cheers

Tim