Why are our native birds ill-equipped to deal with introduced mammalian predators?

Prior to the introduction of mammalian predators like stoats and rats, there were only avian predators in New Zealand eg the Haast eagle. Avian predators hunted by sight, so their prey adapted - kiwi became nocturnal as they could not be seen at night, while the kakapo developed camouflage green/brown plumage that blended in with their surroundings.

However when stoats and rats arrived that hunted with sight, smell and sound our native birds were defenceless - their numbers rapidly decreased and their ranges became restricted to the few places stoats and rats were either not present or not in high numbers – hence the value of our present offshore island refuges such as Hauturu Little Barrier Island.