Hi caffeine, it's just some thoughts I had, not a complete answer.
I'm not quite sure I grasp the point you're making with the starlings.
I just figured they were a good example of what you were describing.
... but this doesn't seem to tell us much about whether they'd trade their adaptability for specialisation if stuck in an unchanging environment for tens or hundreds of thousands of years.
It would be interesting to see if they still mate with european birds with the same degree of success as is normal in each subpopulation (neutral drift).
One of the problems with a highly adaptable species with a large range of possible habitats would be confining it to a stasis ecology.
To study the idea you'd probably need to set up a lab experiment, first to select for multiple ecology adaptation, then to isolate parts in each sub-ecology.
I'd bet on some traits being lost by drift if they are not selected against.
Enjoy.
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