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Back-evolution of limbs.

I don't know whether the following has been discussed before, but:

Why is it that birds--and only flying birds at that--are the only
creatures to return to their former mode of locomotion, i.e., they may
become secondarily flightless.

I haven't heard of a bat or flying squirrel doing such a thing.

And I don't know of any swimming mammal or bird which has returned to a
terrestrial existence.  Why, for example, has no seal colonized some
competition and predator-free island and redeveloped structures for

Perhaps there is a unidirectionality to limb evolution.

Or, more likely, the evolution of these structures requires such a long
period of time that it is unlikely to happen in the relatively short
geological moments in the life time of islands.  And on mainlands 
pre-existing species also don't allow the luxury of such development time.

So a bird loses the ability to fly--either because it is more likely to
get blown off an island, or because large size becomes advantageous, or
because walking is cheaper in its new environment--what does it do with
its appendages.  They atrophy, right?  The bird has long before "learned"
to use its bill and feet to manipulate food and to fight.

I wonder if this apparent unidirectionality to limb development (if it
exists) has any ramifications for the current dino-bird debate.  That is,
something that once flew should not, in the presence of competition and in
the absence of a _very_ long time, should not redevelop fully functional