RE: Dromornithids and size limits.

["Eric Martichuski" <herewiss13@hotmail.com>]

Ok.  This is getting slightly heated.

1) My bad for tetrapod.  I got tired of writing quadruped, since I'm not 
entirely confident in my spelling of it.  Whatever the word, there is, 
however, an inherent structural difference between four _legged_ creatures 
(reptiles, mammals, whatever) and bipedal, winged birds.  That's what I was 
trying to convey.  A wing is such an intensively modified limb, that it 
ought to be looked at (in this debate) as a seperate structure.  The 
homology is still there, but it's _extremely_ distorted/modified. In any 
case,  I would hope that my meaning was fairly clear, regardless (though I 
acknowledge the importance of proper vocabulary).

2) I'm not making a _value_ judgement about birds!  Birds are awesome, there 
are more bird species than mammal species, etc., etc.  But birds aren't 
perfect.  Neither are mammals, but I haven't been concerning myself with the 
ludicrous idea of a mammal trying to horn into an avian niche.  I could, but 
it seems even more obvious, and I believe the initial topic had to do with 
flightless birds.  So talking about mammalian inferiority in regard to bird 
niches, is really just confusing the issue.  It's not relevant.

It's a simple question:  _Does_ a wing contain the same adaptive flexibility 
as a generic quadruped/tetrapod limb?

Obviously, given time, it does.  But I hold that the generic limb will 
almost always get there faster because it's in a "neutral" state (or closer 
thereto).  The generic quadruped limb is equidistant from several peaks of 
fitness, while the highly evolved and specialized wing is _not_.  In order 
to reach another niche, the bird has has to climb down from its _own_ peak 
first.  Meanwhile, the quadruped already has the opportunity to start 
climbing the new slope.  That's all.

It's not a totally hard-and-fast rule (what is in biology?), but it seems a 
common-sensical enough observation.

In order to have the sky (not a bad thing, really) birds paid the price of 
losing adaptive flexibility in it's forelimbs.  That's just what 
specialization does, and you cannot argue that birds are _highly_ 
specialized for their aerial niche.  It is, in fact, more specialized for 
it's niche than virtually any other quadruped limb.  There is more 
anatomical  similarity between a seal's flipper and a lizard's foot than 
there is between a bird's wing and virtually _any_ other tetrapod limb.

3) When I use the terms "inferior" and "superior" I'm refering to fitness, 
as in differential reproductive success.  A mole's adaptations to digging 
are superior to those of a burrowing owl.  And the mole's adaptations to 
flight (or sight, or predation, etc.) are _vastly_ inferior to those of the 
owl.

In that a quadruped has two more apendages available for _any_ adaptation 
than a bird does (or at least, two more apendages more readily _accessible_ 
for adaptation), the quadruped is superior in regards of adaptive 
flexibility.  It is more "fit" to exploit new, non-aerial niches.

It's not "Birds suck!  Mammals rule!"  The arguments I'm making could apply 
equally well to reptiles or amphibians.  And I'm conceding that Birds own 
the sky (not that it _needs_ much conceding).  That's a lot of niches in its 
own right, none of which a quadruped is more "fit" to reach and occupy.

4)  I make no reference to intelligence either.  Since it doesn't, as you 
point out, equate with manipulative ability, so why are you bothering to 
bring it up at all?  An African Grey Parrot is a genius, but it still has 
forelimbs that are so highly modified that adapting them to another function 
would be at best, slow, and at worst, difficult compared to the same 
adaptive process applied to a generic quadruped limb.

Manipulation isn't even the right criteria.  A mole has virtually no 
manipulative ability at all!  Still, reaching the mole's level of fitness, 
in regards to earth-moving, will be a faster, more effective, process for a 
generic non-subterranean mammal's limb than it would be with a bird wing 
undergoing the same adaptive changes.

If, as you seem to argue, birds can compensate and do it all bipedally, that 
still leaves the fact that they've tossed out half their adaptive potential 
by turning their wings into tiny, hidden stubs (since we're referring solely 
to flightless birds).  It's that adaptive potential that I refer to.  It's 
the same idea referred to with polyploidy (if I have the term right).  The 
extra chromosomes, by their very existence, allow for more adaptive 
potential.

5.
> 1. No mammal has successfully invaded a typically avian habitat, nor has
> a bird, that we know of, have done the same to a mammalian habitat.

This is pretty much what I've been saying, actually.  While I'm not 
concerning myself with the invasion of avian niches, the exact same argument 
applies.  If competition existed, Birds would arrive at any specific winged 
niche first because they've already _got_ the wing, while mammals would have 
to "invent" it before moving to further specialization.  Applied to the 
reverse case, the bird would first have to _de_-adapt its wing.

_A_ reason we don't see a niche war is because it would never get that far.  
I believe the term is competitive exclusion.  The niches are filled and the 
current occupants are _much_ better that their jobs than any other species 
trying to evolve in that direction.  So any species attempting to switch 
from aerial to terrestrial, or vice versa, is heavily penalized in terms of 
fitness compared to the species around it on either side.  And if a new, 
terrestrial, niche opens up, quadrupeds are better placed (all things being 
equal) to exploit it first.

That's why most flightless birds (with the exception of 3-4 large, fast, 
species) are found on islands devoid of virtually _all_ quadrupeds.  When 
competition begins with human-introduced mammals, the mammals eventually 
take over the formerly terrestrial avian niches (though I admit that this 
have more to do with a general "un-preparadeness" on the part of all island 
species).

Can you see now that I'm not trying to defame birds?  "Fish gotta swim/Birds 
gotta fly" and four limbs allow more scope for adaptive flexibility than 
two.  That's just how it works.

Eric
______________________________________________________________________
"There is no other wisdom,
And no other hope for us
But that we grow wise. -- Diane Duane
______________________________________________________________________

_________________________________________________________________
Tax headache? MSN Money provides relief with tax tips, tools, IRS forms and 
more! http://moneycentral.msn.com/tax/workshop/welcome.asp