[Senator Bedfellow]

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.

Which is really quite a good hypothesis, in the end ;)

[CarolinaGuitarman]

Especially on islands, where flying can be dangerous with winds blowing you out to sea. Flightless insects (flies and such that had vestigial wings) are common on islands.