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>Apologies if this has already been stated, but I thought secondary 
>flightlessness evolves where there is a lack of predation?  Both island 
>isolation and large size (larger than any other animal that might be a

>anyway) could fit this description. Not sure if you were saying that large
>equals too much weight to fly (which I don't care to dispute), but maybe 
>Phorusrhacoids had no need to fly, because there was nobody else mean enough
>fly away from?
>Curtis Olson

I think it is certainly fair to say that no flying animal will evolve
flightlessness if that condition is selectively disadvantageous!  Presumably
access to food or shelter would not be a factor preventing loss of flight
because any bird that becomes incapable of flight would presumably have given
up flight as all but a very rarely-used behaviour well before it lost the
physical ability to do so.  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).  There are certainly a
fair number of birds around today that can fly but rarely do so.
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