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I would like to add to Ron Orenstein's post responding to Curtis Olson's.
 On Wed, 5 Aug 1998, Ronald Orenstein wrote:

> I think it is certainly fair to say that no flying animal will evolve
> flightlessness if that condition is selectively disadvantageous!

I would go further and say there must be active selection for flightedness
to maintain it.  It is expensive.  Presumably the advantages it affords
are significant in certain niches.

> The only good reason I can think of for retaining
> the ability to fly but using it very seldom would be to escape predators, or
> just possibly to reach a roost site out of their reach (and some flightless
> birds can do that anyway by leaping and/or climbing).

How about dispersal ability?  Over geological time rails may have met
with success because they have these roll-of-the-dice migrations, i.e., 
they lack some of the site fidelity possessd by waterfowl.

Migration?  Waterfowl tend to fly less than other birds.  They don't need
to do it to forage--I mean they don't capture prey on the wing and their
food is usually on the ground.  The nene (pronounced "nay nay")
illustrates several points.  Compared to other black geese, the nene's
wing muscles and bones have been reduced 16%.  This is probably because it
doesn't migrate.  We may have caught the nene on its way to flightlessness
(though, as Ron noted, it still flies plenty).  An extinct goose's fossil
was found on the island of Molokai.  It was bigger and its wings were
stunted.  It was probably flightless.  So these migratory birds have a
good potential for being isolated and when they are, they may well become
flightless.  Already, the nene has the smallest range of any goose 
species.  However, an essential precondition is that there be reduced
predator density.  Mammals recently brought to Hawaii, particularly
mongoose--no kind of goose, that--are doing a number
on the nene's nests. 

John Bois.