RE: Dromornithids and size limits.

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

> From: "Jaime A. Headden" <qilongia@yahoo.com>
> Some birds do precisely that, standing on one leg and using the other to
> play with things.

Are these ground or arboreal birds?  Being able to use one foot while the 
other is tightly gripping a tree branch isn't quite the same as _balancing_ 
on one leg while using the other.


> However, parrots and turacos are birds that are effectively three-legged 
> climbers, in that they use >their beaks to manipulate and clamb with.

Effectively, yes.  But are they _effective_ climbers?  Aren't most 
tree-adapted quadrupeds likely to be faster, and more agile?  Plus, they 
have their jaws free.  Can a parrot climb and hold a nut in its beak at the 
same time?  Well, maybe it could, but it'd still be clumsy.

> Many corvoids and parrots use tools held in the beak.

Quite true.  But _how_much_better_ can they improve?  Take, as 
representatives, a sparrow and a mouse.  Subject them to intense selection 
pressure for tool manipulation.  The sparrow will come to do a _lot_ with 
its beak, but I doubt it'll be able to match the mouse with its two limbs of 
manipulators.

> Birds that need to manipulate develop a means, usually in the head.

They certainly do.  All I'm saying is that a tetrapod is, all things being 
equal, better poised to develop a _superior_ means.  Birds usual maintain 
their edge despite "inferior" adaptations precisely because of their wings.  
Lose flight and get stuck with the "inferior".  Some niches _will_ remain 
open for obligate terrestrial birds, but they don't have the evolutionary 
_pathways_ to explore as many niches as tetrapods...or at least, not explore 
them as quickly and effectively as a tetrapod itself.

> If they were to lose flight, for instance, and specialize as fossorial, it 
> is likely that they...would
> specialize for it. The need to specialize for burrowing is, to a point, 
> unneccesary.

In their current niches, yes.  If a flightless bird _attempted_ to enter the 
fossorial niche, all things being equal, it would find itself at a great 
disadvantage against a tetrapod entering that niche _at_the_same_time.  
Because a generalized tetrapod is already a better excavator than a 
generalized biped.  If you're dealing with a flightless bird who's been so 
for some time, so that the wings have atrophied to almost nothing (like 
kiwis), it's at an even _bigger_ disadvantage because bringing the wings 
into play as shovels requires bringing them back as _limbs_ in the first 
place.

> Meanwhile, larger birds like phorusrhacoids have HUGE heads for rending...

Meanwhile, while the Killer Bird has one predatory appendage, the 
sabretooth, or other predator has _three_.  A lion leaping up to claw and 
bite as the hindquarters of its fleeing prey has a better chance of staying 
attached than a phorusrhacoid would with its single, albeit wickedly hooked, 
anchor into the prey's flesh.  _In_general_ a tetrapod predator has an 
advantage over a biped predator.

> No mammal niche is threatened by birds.

Exactly.  I think we might actually agree here.  It's just a matter of 
interpretation.  You say they _aren't_ threatened.  I say they _can't_ be 
threatened.  Because, within those niches, tetrapods are better suited to 
develop the requisite adaptations.

I say nothing against the marvelous talents of birds.  They rule the skies 
with an awesome diversity and fecundity.  I merely say that, having 
specialized to such an amazing degree, they pay the price of finding it more 
difficult to de-specialize and find something else.  They are, in a sense, 
stuck.

That's all.

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

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