hard · Gre Verbal

Naturalist: The blue-throated warbler and the gray-crowned warbler occupy the same forest, yet the blue-throated species has declined sharply over the past decade while the gray-crowned species has thrived. Both eat the same insects and nest in the same trees. The only notable difference is that blue-throated warblers migrate south for the winter, whereas gray-crowned warblers remain in the forest year-round. Therefore, whatever is causing the blue-throated decline must be occurring outside the forest during the winter migration. Which of the following is an assumption on which the naturalist's argument depends? A) The insects that both warblers eat have become less abundant in the forest over the past decade. B) Gray-crowned warblers would decline if they were forced to migrate south for the winter. C) The blue-throated warbler's wintering grounds have suffered greater habitat loss than the forest has. D) No factor within the forest affects the two warbler species differently despite their shared diet and nesting sites. E) Other migratory bird species that share the warblers' wintering grounds have also declined.

1. The insects that both warbler species feed on have grown markedly less abundant across the forest over the past decade.
2. Gray-crowned warblers would themselves go into steep decline if they too were forced to migrate south each winter.
3. The blue-throated warbler's distant wintering grounds have suffered greater habitat loss over the decade than the shared home forest has.
4. No factor within the forest affects the two warbler species differently despite their shared diet and nesting sites.
5. Other migratory bird species that share the warbler's wintering grounds have also declined in number over the same decade.

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