Most of the year, the hermit thrush, a North American songbird, eats a diet consisting mainly of insects, but in...

GMAT Critical Reasoning : (CR) Questions

Source: Official Guide

Critical Reasoning

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Most of the year, the hermit thrush, a North American songbird, eats a diet consisting mainly of insects, but in autumn, as the thrushes migrate to their Central and South American wintering grounds, they feed almost exclusively on wild berries. Wild berries, however, are not as rich in calories as insects, yet thrushes need to consume plenty of calories in order to complete their migration. One possible explanation is that berries contain other nutrients that thrushes need for migration and that insects lack.

Which of the following, if true, most seriously calls into question the explanation given for the thrush's diet during migration?

Hermit thrushes, if undernourished, are unable to complete their autumn migration before the onset of winter.

Insect species contain certain nutrients that are not found in wild berries.

For songbirds, catching insects requires the expenditure of significantly more calories than eating wild berries does.

Along the hermit thrushes' migration routes, insects are abundant throughout the migration season.

There are some species of wild berries that hermit thrushes generally do not eat, even though these berry species are exceptionally rich in calories.

Solution

Passage Analysis:

Text from Passage	Analysis
Most of the year, the hermit thrush, a North American songbird, eats a diet consisting mainly of insects	What it says: Hermit thrushes normally eat insects most of the time What it does: Sets up the typical diet pattern as baseline information What it is: Author's factual statement about normal behavior
but in autumn, as the thrushes migrate to their Central and South American wintering grounds, they feed almost exclusively on wild berries	What it says: During migration in fall, thrushes switch to eating mostly berries instead What it does: Shows a dramatic shift from the normal diet pattern we just learned What it is: Author's factual statement about seasonal behavior change Visualization: Normal year: 80% insects, 20% other → Migration time: 90% berries, 10% other
Wild berries, however, are not as rich in calories as insects	What it says: Berries have fewer calories than insects What it does: Creates a puzzle by showing the diet switch seems nutritionally worse What it is: Author's factual comparison Visualization: Insects: 100 calories per serving → Berries: 60 calories per serving
yet thrushes need to consume plenty of calories in order to complete their migration	What it says: Migration requires lots of calories to succeed What it does: Deepens the puzzle by emphasizing why the lower-calorie choice seems wrong What it is: Author's factual statement about migration energy needs
One possible explanation is that berries contain other nutrients that thrushes need for migration and that insects lack	What it says: Maybe berries have special nutrients for migration that insects don't have What it does: Offers a solution to resolve the calorie puzzle we've been building What it is: Author's proposed explanation

Argument Flow:

The passage starts with normal thrush behavior, then presents a puzzling seasonal change. It builds tension by showing this change seems nutritionally disadvantageous (fewer calories when more are needed), then offers a potential solution to explain the puzzle.

Main Conclusion:

Berries might contain special migration nutrients that insects lack, which could explain why thrushes switch to this lower-calorie food during migration.

Logical Structure:

The argument uses a problem-solution structure: it establishes a behavioral puzzle (switching to lower-calorie food when high calories are needed) and then proposes a hypothesis (special nutrients in berries) to resolve the apparent contradiction.

Prethinking:

Question type:

Weaken - We need to find information that reduces our belief in the proposed explanation that berries contain special nutrients for migration that insects lack

Precision of Claims

The explanation specifically claims that (1) berries contain other nutrients needed for migration and (2) insects lack these nutrients. We need to attack either or both of these precise claims.

Strategy

Since this is a weaken question, we want to find information that makes the proposed explanation less believable. The explanation tries to solve the puzzle of why thrushes switch to lower-calorie berries during migration. We can weaken this by showing alternative reasons for the diet switch that don't require special nutrients, or by showing that the nutrient explanation doesn't actually make sense.

Answer Choices Explained

Hermit thrushes, if undernourished, are unable to complete their autumn migration before the onset of winter.

This actually supports the importance of proper nutrition during migration rather than weakening the explanation. It reinforces that thrushes need adequate nutrition for migration, which makes the proposed explanation about special nutrients in berries seem more plausible, not less. This doesn't give us any reason to doubt the berries-have-special-nutrients theory.

Insect species contain certain nutrients that are not found in wild berries.

This doesn't weaken the explanation at all. The proposed explanation specifically claims that berries contain nutrients that insects lack - this choice talks about the reverse situation (insects having nutrients berries lack). Both could be true simultaneously, so this doesn't create any contradiction with the given explanation.

For songbirds, catching insects requires the expenditure of significantly more calories than eating wild berries does.

This seriously weakens the explanation by providing a compelling alternative reason for the diet switch. If hunting insects burns way more energy than eating berries, then thrushes might switch to berries simply for better net calorie efficiency during migration - not because berries contain special nutrients. Even though berries have fewer calories per item, the energy savings from not having to hunt could result in better overall energy balance. This gives us a completely different explanation that doesn't require special nutrients at all.

Along the hermit thrushes' migration routes, insects are abundant throughout the migration season.

This strengthens the puzzle rather than weakening the explanation. If insects are readily available during migration, it makes the switch to lower-calorie berries even more mysterious, which actually makes the special-nutrients explanation seem more necessary and plausible.

There are some species of wild berries that hermit thrushes generally do not eat, even though these berry species are exceptionally rich in calories.

This doesn't significantly weaken the explanation. The thrushes might avoid these particular high-calorie berries because they lack the specific nutrients needed for migration, or because they're harder to digest, or for other reasons. This choice is consistent with thrushes being selective about berries based on nutritional needs beyond just calories.

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