Passage Summary: Cardinals used to be rare in a cold area, but now they are everywhere; since it got a bit warmer during that same time, the heat must be why they moved in.

Conclusion: The rise in average winter temperatures is likely the cause of the northern cardinal's population growth in Nova Scotia.

Reasoning: Cardinals were rare in 1980 when it was colder but became common by 2000 as the average winter temperature increased.

Analysis: This argument relies on a classic correlation-to-causation jump, assuming that because the temperature rose while the birds arrived, the temperature caused the arrival. To weaken this, we need to find an alternative explanation for the cardinals' success that has nothing to do with the weather. Perhaps a new type of bird feeder became popular, or a major predator of the cardinal went extinct in that region. Any evidence suggesting the birds adapted to the cold or that another environmental factor changed would effectively undermine the author's claim.