Geography
Why do island birds so often lose the ability to fly?
- There is nothing to escape from
- The winds are stronger
- There is less food
The answer is There is nothing to escape from. Flight is expensive, and with no ground predators it is dropped.
Flight is enormously expensive. The muscles that power it can be a quarter of a bird's body weight and consume energy accordingly, and the skeleton has to be built light, which limits what else it can do. On an island with no ground predators there is nothing to escape from, and evolution stops paying for a capability nothing is using.
It has happened again and again, independently - rails in particular seem to lose flight almost as soon as they land somewhere safe, and dozens of island species have done it. New Zealand, isolated for 80 million years with no land mammals, produced the kiwi, the kakapo, several moa and a flightless parrot, wren and goose.
The vulnerability is exactly what you would expect. Almost every flightless island bird went extinct within a few centuries of humans arriving with rats, cats, pigs and dogs, because a bird that evolved without ground predators has no instinct for them. The dodo is the standard example and was not stupid, merely unafraid, which is a rational thing to be until it suddenly is not.