I keep an eye on the kakapo project. One of the islands they prepared and thought was relatively possum free turned out to have about 5,000 possums on it. Another island was abandoned when stoats swam to it from the mainland.
It's not only the birds that are in trouble, the seeds of plants in NZ can't survive a trip through the gut of a mammal and in areas with a high level of feral animals the forests are dying.
I counted 9 native bird types while I was there, not counting sea birds.
