Passage Summary: People used to think New Zealand's birds thrived because there were no land mammals to compete with. However, scientists recently found old mammal fossils on the island, leading some to claim the 'no competition' theory is now debunked.

Conclusion: The discovery of ancient mammal fossils in New Zealand proves that the theory attributing the island's unique bird population to a lack of mammalian competition is false.

Reasoning: New Zealand was previously thought to have no native land mammals, but the discovery of primitive land mammal fossils provides the first concrete evidence that they did exist there.

Analysis: The author is making a huge leap by assuming that just because mammals existed at some point in New Zealand's history, they must have coexisted with—and competed against—the bird populations in question. To weaken this, look for an answer that suggests these mammals didn't actually pose a threat or overlap with the birds. Perhaps these mammals went extinct millions of years before the current bird species evolved, or they lived in a way that didn't interfere with the birds at all. If the mammals weren't around to compete, the original theory might still be perfectly valid.