Passage Summary: Scientists found way more bird bones in the old dirt than in the new dirt, so they figured humans must have wiped out the birds when they showed up.

Conclusion: The arrival of humans on Tiliga Island is the primary cause for the significant decrease in bird population and diversity.

Reasoning: Soil layers from the 80,000 years before human arrival show a much higher density and variety of bird bones than the top layer representing the 3,000 years since humans arrived.

Analysis: This argument relies on a 'correlation equals causation' jump, assuming humans are the only possible culprit for the bird decline. To weaken this, we should look for an alternative explanation for why the bird population dropped or why the bone records might be misleading. Perhaps a massive climate shift occurred 3,000 years ago, or maybe the soil conditions changed such that bones from the last 3,000 years didn't preserve as well as the older ones. We want to find a fact that suggests the birds were in trouble regardless of the human neighbors.