On the BBC podcast "Best of Natural History Radio," I heard David Attenborough say
Flying is a very energetic business, much more so than walking or running, and birds don't fly unless they have to. Indeed, they stop doing so if it's safe to do that. The process has happened in recent time on islands where there are no ground predators. So cormorants on the Galapagos and rails on Central Pacific islands have stunted wings that are no longer big enough to get the bird into the air.
But I would tweak the perspective on that and say, not that you don't fly unless you have to, but that you can't fly, unless you have to. I like that juxtaposition of wretched necessity and flying.