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RE: BCF ANDPDW




On Wed, 5 Aug 1998, Ronald Orenstein wrote:

>...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.

That is true.  However, on a predatorless island such selective pressures
are likely to be trivial compared to those which once maintained migratory
ability.  Indeed, they may be subsumed by other selective pressures, for
example the need to be big to defend more valuable island territory.

> 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.

But all this assumes diet choice to be the prime selective factor (in
absence of migratory selection).  On islands, species are more likely to
be able to _walk_ to the food (this is a generalization--I realize Hawaii
is a big island).

> 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.

I agree that the move to flightlessness is contingent.  In the nene's
case, flight may well be maintained by diet choice.  But again, for the
fossil bird, once the tyranny of maintaining migratory ability was ended
some other force may have driven the bird towards flightlessness.  I'm not
sure it is possible to predict anything other than: flightlessness is more
likely on an islands due to lack of predators.

> 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.

I'm not sure I understand this.  Flightlessness is more common on
islands, I thought?