So can overgeneralization, though - it's the answer to the question of why there are no bears in Africa. There used to be, though, several million years ago. But bears are generalists, jacks-of-all-trades and masters of none. They don't hunt as efficiently as lions and crocodiles, and they don't forage as efficiently as zebras and giraffes. So they got pushed out by the specialists.
By the same token, it's difficult to envision a body plan that would allow a bird to both fly as well as an albatross and swim as well as a penguin. More likely, such a bird would be sort of mediocre at both. Or pretty good at one, and not really good at all at the other. Given the right environment - bears in Alaska - being sort of mediocre at both might be enough to constitute a winning hand. Given the wrong environment - bears in Africa - it might be a total loser.
The irrational brigade has managed to toss your excellent thread into the smokey backroom. Never fear. The intellectual value of this thread will persist. I shall henceforth vigorously point out the intellectual connections between Adam Smith, Charles Darwin, and the Founding Fathers. If the creationists squeal, it's because their totalitarian worldview is exposed for all to see.
My point is a narrow one. In the case of birds, the hypothesis is that if the flight skill is not necessary for survival, it might consume energy unproductively, and might be selected against. I will tap into the clan gene pool on this one, to attempt to selfishly gain for the selfish gene, a competitive advantage, when the reposity of necessary section of the pool returns from Asia, where the reposity is leading, yup you guessed it, a birding tour.
Are the flyers/swimmers going extinct faster than the flightless birds or vice versa?