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RE: BCF ANDPDW



>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).

Well, let's just say that they are less likely to run into trouble if they
do. 
However, this gets complicated - you have to consider what new niches might be
opened up on an island that would favour a flightless bird.  You also have to
examine why evolution of flightlessness involves, usually, a lot more than the
loss of flying ability, including changes in bill and body size and
proportions.

The issue may not be, or may not only be, lack of predators but an absence of
terrestrial competitors and the trends in plant evolution on islands (see, for
example, Sherwin Carlquist's "Island Life").  For example - there is a
tendency
for oceanic island plants to produce larger, heavier seeds and fruit than
their
mainland relatives (possibly to reduce dispersal ability for seeds that would
be more likely, in their original state, to be blown out to sea and lost. 
Combine the presence of heavier seeds and fruits with the absence of mammalian
terrestrial foragers on fallen fruit (such as, for example, agoutis or pacas),
and you have a niche for a ground-feeding bird with a large, stout bill, a
good-sized gizzard etc.  This fits the Dodo, the Hawaiian flightless geese
etc.

>> 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?

My point was that to evolve into a flightless island bird requires that you
get
to the island in the first place, and this is more likely to happen for a
strong long-distance flyer than it is for a weak-flying, sedentary species.
--
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