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RE: Dromornithids and size limits.
From: "Jaime A. Headden" <firstname.lastname@example.org>
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
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,
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
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,
"There is no other wisdom,
And no other hope for us
But that we grow wise. -- Diane Duane
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