Just come back from the vets and read a good article. This zoologist based in the Hutt studied his cats hunting habits over 18 years and came to the conclusion that his cat actually benefited Native Bird life by keeping the rodent population down, the rodents do more damage than cats He also stated letting your cat out at night actually helped also as birds are not active but rodents are, increasing the likelihood that cats will hunt rodents.
Very small sample size. Other studies with larger sample sizes have come to different conclusions: http://sciblogs.co.nz/politecol/?tag=cat
Yeah my own experiences back his up however. My cats have bought in very few birds in but quickly wiped out the rabbit population and usually bring in mice and the odd rat.
I think the damage cats do would be more pronounced on rodent free reserves.