That is most interesting. I was not aware that the humeral strength of
grebes overlapped with the most basal birds. I wonder how Confuciusornis
would fare using that same metric.
I also wonder if one measures the strength of the humerus by diameter, by
wall thickness, by density, by putting specimens in a press or so on. I
imagine it could be complicated. I've read that Rogers and LaBarbera
(Journal of Zoology, 230, issue 3, pg. 433 1993) demonstrated that the
density of trabeculae inside the pigeon humerus also affects mechanical
strength. I ask because it is easy to snap a pigeon humerus in the lab,
but not so easy to do with a fossil.
By the way I completely concur that comparing Archaeopteryx to a
hummingbird makes the former look radically underbuilt. I am keenly aware
that the minimum requirements for flight are way lower, in fact that was
the point I was trying to make, however poorly.
I would like to see how the humerus of Geococcyx stacks up. Meinzer (The
Roadrunner, 1993, Texas Tech U. Press) reports that they usually ascend to
their nests in trees by leaping upward to a series of branches, and not by
flying. I've been imagining similar behavior in an animal like Anchiornis
or other small troodontids - with long legs and probably somewhat
underpowered flight apparatus. One could imagine how an evolutionary
feedback loop could be set up in an animal that is habitually leaping in
and out of trees like this. And until I read Meinzer's report I never
really imagined an animal the size of a roadrunner leaping eight feet into
a tree. But now I can imagine these small, basal, paravians doing so.
Perhaps the ancestral Paravian didn't need to be strictly arboreal before
one or more lineages began to glide, they could have just been roosting or
foraging part-time in trees.
Moreover, we would both agree that 20% doesn't matter to extant
birds with highly derived flight apparatus, but modern birds have a lot
spare capacity. They can stoop at 200 mph, hover in mid air, and fly
thousands of miles! In an animal that is barely capable of aerodynamic
like any hypothetical ancestor of birds, 20% could be a crucial
between ascending flight and gliding.>
One point worth mentioning here is that while some modern birds do indeed
execute high-load maneuvers, those that do also have stronger limbs than
those that do not. Peregrine falcons, for example, have humeri over 6
times stronger than those of an albatross, relative to body mass (i.e.
they can take 6+ times as many body weights of force). Therefore, one of
the key methodological points in comparing the structural strengths of
feathers or bones in living birds to fossil ones is to use relevant living
taxa - don't use hummingbirds and falcons as comparisons for
Archaeopteryx, for example. Doing so will inevitably make Archaeopteryx
look underbuilt. By contrast, something like a grebe, which is still
quite a derived and reasonable flyer, does not have limbs all that much
stronger than the most basal birds, relative to mass (I am using the
Yalden mass for Archie in that comparison).
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