Most of the year, the hermit thrush, a North American songbird, eats a

Let's review the situation:

• The birds need plenty of calories in order to complete their migration.
• Berries are not as rich in calories as insects.
• If the birds need lots of calories and insects are higher in calories, why do the birds eat berries during migration instead of insects??
• "One possible explanation is that berries contain other nutrients that thrushes need for migration and that insects lack."

We are looking for an answer choice that calls this explanation into question. We don't need to prove that the explanation in the passage is false. However, if one of the answer choices provides a plausible alternative explanation, then that would cause us to doubt the explanation given in the passage.


Remember, the birds need plenty of calories. We know that insects are higher in calories, but now we are told that the birds expend significantly more calories getting those calories from the insects. So even though the insects are high in calories, the birds have to waste a lot of calories catching the insects. If the goal is to achieve the highest net balance of calories, then the birds might be better off eating the berries.

Again, we don't need to prove that eating berries will lead to a higher net balance of calories. But this is certainly a plausible explanation for the phenomenon described in the passage. Since we now have a plausible alternative explanation, the explanation given in the passage is certainly called into question. (C) looks good.


We are trying to explain why the birds eat berries instead of insects. If the insects were NOT abundant, that might explain why the birds eat berries. But if the insects ARE abundant, then the birds should have no problem finding them. So why are they eating berries instead? Choice (D) does not explain the apparent discrepancy, so eliminate this one.


(E) simply tells us that the birds avoid some high-calorie berries. But we already know that the birds eat berries during migration and that the berries are lower in calories than insects. (E) does not explain why the birds chose the lower-calorie option, so eliminate this one.

(C) is the best answer.