Passage Summary: We have a lot of birds this year because last winter was warm. The warmth meant they didn't have to fly south—where many usually die—and they could find food easily on their own.

Conclusion: The unusually large bird population this year is the result of the previous mild winter.

Reasoning: The mild winter allowed birds to find food naturally and stay in their summer ranges, which prevented the high death rates usually associated with migration.

Analysis: This is a causal argument, so we want to strengthen the link between the mild winter and the population boom. A great way to do this is to rule out other potential causes, such as an unusually productive breeding season or a decrease in predators. We could also strengthen it by showing that the 'migration attrition' mentioned is a significant factor in bird mortality. Look for an answer that confirms the mild winter's specific effects or eliminates an alternative explanation for the population increase.