[flippyhead]

What I've understood is that if the change that's detrimental is compensated for by other attributes that do connote fitness this can be an evolutionarily stable strategy. That is, if the the bird is able to survive even with the club wings that is in fact an indication of relative fitness and the club wings can be selected for. Though I've only heard this theory in reference to things like bulking/attractive plumage that itself connotes some kind of fitness. For example, those birds with crazy long tails are arguably less able to hunt or whatever but the ones that do hunt effectively cause the female to think: wow so attractive! And he can manage to provide.