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>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.


>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.

I did not include this on the grounds that the ability to disperse over long
distances will probably be lost well before the final transition is made from
limited flight ability to flightlessness - in effect, a bird may have no
selective pressure re the kind of flight required for dispersability but still
face selection pressure to maintain the ability to flutter for short
and it is the relaxation of that latter pressure (and/or pressure towards
flightlessness) that will result in the final loss.

Also, I doubt very much that (except for long-distance migrants)
itself can be selected for (at least the long-distance kind that gets rails
to oceanic islands) - what is selected for is the kind of flying ability that
can lead, in some circumstances, to dispersal.  Though I think that is what

>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.

Again, loss of migratory ability can happen long before flying ability is
lost.  You can see the morphological effects - eg in the Variegated Flycatcher
of South America, southern migrant populations have longer wings than northern
non-migrants.  This sort of difference has nothing to do with the loss of the
ability to fly altogether.

  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. 

I cannot argue with this, but again I point out that the factors leading to
loss of migratory ability have probably ceased to have any selective effect by
the time the bird gives up flying altogether.  If the nene ever does become
flightless it will be because other factors are in play, since there is no
longer any migration to act as a selective filter.  I suspect in the case of
the fossil geese on Hawaii there was an evolutionary advantage to increasing
size and robustness; this may not be the case for the nene, which is a fairly
small goose, possibly because of its food choices.  So the nene could become
flightless, or it could stay just as it is - but I would not expect the former
unless its diet shifted to give an advantage to larger birds.

Of course this will probably never happen now because of the human changes on
Hawaii.  I suspect the fossil birds lived in very different, probably lowland,
habitats from that of the nene.  As a bird of uplands on a volcanically active
island, where feeding sites may be islands of vegetation (or "kipukas")
separated by extensive lava flows difficult to cross on foot, the nene may in
any case be under active selection pressure to retain flying ability to move
easily from site to site, or to join flocking associations in the non-breeding
season.  If this is so it may not be "on its way to flightlessness" at all.

 So these migratory birds have a
>good potential for being isolated and when they are, they may well become

Again making the point that the kind of selection pressure leading to loss of
dispersal ability may actually make it LESS likely that flightlessness will
occur, at least on islands, because the birds with weaker flight won't get to
the islands in the first place.  Note, for example, the complete absence of
huge neotropical bird families that include antbirds, ovenbirds and
woodcreepers on oceanic islands like the Galapagos and the West Indies.
Ronald I. Orenstein                           Phone: (905) 820-7886
International Wildlife Coalition              Fax/Modem: (905) 569-0116
1825 Shady Creek Court                 
Mississauga, Ontario, Canada L5L 3W2          mailto:ornstn@inforamp.net