Re: Pelagornis chilensis

[Mike Habib <habib@jhmi.edu>]

I have now had a chance to read through the new Pelagornis paper from JVP, and 
drool over the exceptional specimen.  It's a very nice paper with some 
wonderful description and analysis.  I do, however, have some skepticism 
regarding the body weight and flight comments.  I am interested to know what 
other list members think of the same:

>From the paper:

"There exists a correlation between the mass of a bird and the least 
circumferences of the femur and tibiotarsus, with log M = 2.411Âlog CFâ0.065 
and logM= 2.424Âlog CT + 0.076, where M is the body mass in gram, CF the least 
femur circumference, and CT the least tibiotarsus circumference (Campbell 
andMarcus, 1992). With least shaft circumferences of 58.4 and 64.1 mm for the 
femur and tibiotarsus of the Chilean pelagornithid, this results in mass 
estimates of 15.6 and 28.6 kg, respectively. Even the larger of these values is 
much less than the estimated mass of 71.9 kg for A. magnificens (Campbell and 
Marcus, 1992), and not significantly above the mass of the heaviest extant 
volant bird, the Mute Swan, Cygnus olor, whose males can reach â20 kg. These 
low values are nevertheless plausible, because the bones of pelagornithids were 
exceedingly thin-walled, and the hind limbs, which had to bear the weight of 
the bird, are very small. In combination with the very narrow wings, these low 
weight estimates testify highly efficient soaring abilities of pelagornithids, 
which appear to have been among the most proficient avian long-distance 
soarers."

I find the mass estimate immediately suspicious because, as the authors note, 
it is within the range for living birds, despite being reconstructed for a 5m 
span animal.  The assertion that the thin-walled bones make this plausible is 
weak, because other authors (Prange et al, 1979; Cubo and Casinos, 1994) have 
shown that thin-walled bones do not correlate with lighter skeletons in birds.  
The observation that the hind limbs are small is also not a convincing argument 
for low mass because quite a few very heavy birds have small hind limbs - the 
short length gives them a shorter moment arm which partially offsets the 
smaller diameter.  As such, albatrosses and sulids, for example, have hind 
limbs only slightly weaker than average for a bird for their mass, despite 
having very small hind limbs.  In extreme cases (Phaethon and Fregata), the 
hind limbs are truly very weak.  Furthermore, limb strength correlates not only 
with total body mass, but also with behavior and overall locomotor dynamic, so 
for example, oceanic soaring birds tend to have weaker hind limbs than inland 
soaring birds of the same mass.  The argument that the low mass improves 
long-distance soaring ability is also a concern - low wing loadings can improve 
loitering performance (to a point), but long-distance soaring works best at 
higher wing loadings.  Once again, the concept that "ultra light is ultra 
efficient' rears its head.  Sigh.  So it goes.

In the conclusions the authors suggest that: "Because rotation of the humerus 
of pelagornithids was restricted, takeoff may have been by simple spread of the
wings against headwinds" - this is not a plausible means of large for a large 
bird, and if the humerus were truly locked against any rotation, launching from 
the ground would essentially be impossible.

Again, I should emphasize that these sorts of myths/misconceptions regarding 
mass and flight in large flying animals are very, very common, so I do not mean 
to aim at Mayr and Rubilar personally.  Rather, it is just a good example of 
the unfortunate literature inertia that exists with regards to understanding 
soaring flight.  The authors did an excellent job of providing details of their 
mass methodology and line of reasoning behind the subsequent conclusions, which 
is why a critique such as this is possible, and is one mark of a good paper.

Cheers,

--Mike


Michael Habib
Assistant Professor of Biology
Chatham University
Woodland Road, Pittsburgh PA  15232
Buhl Hall, Room 226A
mhabib@chatham.edu
(443) 280-0181