Moa

This is interesting:

Skeletal remains and other clues had previously put the moa population in New Zealand at around 159,000 at the time humans arrived, one thousand years ago. But the latest research suggests that there were between 3 and 12 million moa. If both numbers are correct, that means something else decimated the bird population before humans finished it off some 500 years ago.
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Few people doubt that New Zealand’s first human settlers drove the moa to extinction in record time by destroying their habitat with fire and hunting them for food. But something else must have driven the population down from millions to a mere 159,000.

Among the chief suspects, says Gemmell, are disease brought by migrating birds or birds that had been blown off course, and a traumatic geological event.
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The ancient forests are believed to have been impenetrable before foreign herbivores invaded them, thinning the vegetation. But if the new study is correct, then such vast numbers of moa could have had a similar impact on the forest, says Paul Scofield, curator of vertebrate zoology at Canterbury Museum in Christchurch.

“If they are right then a deer-infested forest would be closer to the original New Zealand forests,” he says.