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No universal rules for explaining body size.

In response to Dave's or Collette's list of hypotheses for small size I
submit this slightly different take.  While my view is less general, less
inclusive, it may be more explanative.

On Wed, 6 May 1998, Colette H. Adams wrote:
> But the remark about small birds being diverse has sparked my interest.  In
> ALL taxa that I'm aware of, including dinosaurs, species richness increases
> exponentially with decreasing size.  Why?  The answer is not at all clear
> to me.  It may be that small species tend to have shorter generation times,
> leading to more rapid rates of evolution and speciation.  Or perhaps large
> species have a higher extinction rate.  Or is it a combination of factors?
> I would be interested in hearing opinions.

I'm not sure this is accurate.  Crocodilians, for example, seem to have a
distribution around a higher mean than, say, birds or mammals.  Body size
is, I believe, taxon-dependent.  A ratio can be made  comparing mammals
and birds in this respect (I may be excused for addressing these taxa
since they--birds at least--were the initial prompt for your musings). 
Take a
similarly-sized critter from each--say a turkey and a racoon.  Compare
above turkey/below turkey-size (for birds) with above racoon/below
racoon-size for mammals.  Mammals have a higher
value (isn't that right?).  So again, the question: Why?  
All (most) of the answers given so far appear to apply to both
mammals _and_ birds.  They therefore should be discounted for explaining
the difference _between_ mammals and birds.  Let's check this.  Here
follows a collection of reasons supplied in this thread:

Shorter generation times--both taxa are roughly equivalent.

Higher rates of reproduction--don't know, but roughly equal.

Greater potential for reproductive isolation--winged flight should mean
birds had less potential for this (I mean wings should enhance gene flow) 
and yet they are _smaller_.  But I'm not sure about this point. Birds are
able at becoming reproductively isolated sympatrically--via songs, signals
and such.

Many small creatures in separate niches are more likely to survive than
large homogenous populations--mammals and birds similar in this respect
(but see below).

Higher genetic diversity and evolution rates for smaller
creatures--mammals evolve faster as judged by number of chromosome arm and
number changes per estimated million years!  And yet mammals are bigger.

Higher extinction rate--don't know.

If these factors do not account for size differences between mammals and
birds, what does?

Teeth are better for bigger things--Possibly, but Emus, ostriches, and
rhea are wonderfully adapted. Beaks are awesome tools.  Birds have "teeth"
anyway in their crop.  I have trouble believing that eating
competitions result in either large mammal preeminence, or the
greater diversity in small birds.  From what I've read I think you would
agree with this.  Like you I would be interested in hearing how any
competitor in this respect could affect ostrich distribution.

Bipedalism is a liability for bigger things--Ostriches and company do very
well on two. Indeed, ostrich adults living among the world's most
formidable carnivores suffer little predation.  If predation is not the
issue what is?

Bipedalism is better for small things--doubtful. The smallest things tend
to have more than two (I'm thinking of salamanders, insects, lizards and
such).  Indeed, notwithstanding species such as rails, the great
advantage of two legs for small birds seems, by and large, to be that the
other two appendages are freed up for flight!

Here then are my hypotheses:
Small size in birds relative to mammals is probably due to the
increased number of niches available to that body plan.  Winged flight
enables a bird to forage and then escape to cover.  For example, a small
mammal out of cover risks more than a similarly-sized bird.  The bird can
fly and maneuver away from, say, aerial attack.  Unless close to its
burrow, the mammal must travel and its escape route is
limited to two dimensions (less than the three available to a bird).
Small animals have more hiding places.  But these hiding places may not
occur where there is forage.  The greater range of birds allows them to
both exploit concealment and remote patches.  Conversely, small mammals
are restricted to habitats where the two occur together.

Whethere or not the above is true, considerations such as these (and the
data) call into question the applicability of vertebrate-wide rules for
body size.

Finally, since fewer refuges exist for large forms, a corollary of the
above is that fewer species in any given taxon should be large.  But this
would be wrong.  Again, such conclusions ignore the life-history and
strategic idiosyncracies of each taxon.  Crocodilians, perhaps because
they are nest defenders (perhaps because the only crocs left are
semi-aquatic and so must oviposit in sand near water--making nests easy to
find), are more consistently selected for large size than other taxa.  
However, we may still consider the bird/mammal size differential.   
The following circumstantial evidence suggests that inability to conceal
nests in large oviparous species is responsible for their low diversity.
1. Dramatic increase in large body diversity on predator free islands.
2. The relative immunity of the few mainland large bodied bird adults to
predation and the severe predation on eggs and hatchlings.
3. The relatively quick removal of most large-bodied species from South
America with the interchange.  The only competing hypothesis (no pun
intended) is competition.  And I wholeheartedly share your skepticism that
competition could result in this!
 4. The cooincidence of the evolution of both Savannah grasslands and the
largest phorusrhacoids 27 mya (suggesting that cover and/or productivity
provided by this medium was important).

Whether or not this hypothesis is true, it (and other hypotheses) must
be explored before universal rules are claimed to explain body size (I'm
not saying you are saying they do).