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Re: Flightless bat? Flightless pterosaur?

In response to confidence expressed about bats/pterosaurs ability to
become flightless--especially on remore islands.

Many birds have lost flight function on islands.  A commonsense
explanation for this is that they no longer need to fly from
predators-->wing muscles are expensive to maintain-->w/out selective
advantage for them, other factors weigh heavier, e.g., in times of
little food, the leaner machine is selected (not needing to
maintain--feed--flight muscles).
Surely the same advantages should acrue to bats (or pterosaurs).  So why
so few?  Maybe this has to do with the difficulty of escaping from
vestigiality.  Walking, for bats is vestigial--meaning that while they
still might do it, the flight function has so co-opted their limbs for
flight they may never be efficient walkers.  In birds, rear legs are not
(primarily, at least) involved in flight--they are not vestigial walkers.
And, note that _no_ completely flightless bats or pterosaurs are known.

PS.  I'm not sure "vestigiality" is the right word here.  If not, what is?