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



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