Question 110·Medium·Central Ideas and Details
During a month-long expedition to a series of coastal wetlands, ecologist Ramon Li carefully tracked the movements of several species of migratory shorebirds. He noted that the birds gathered in large numbers on particular mudflats whenever the tide ebbed, areas where his samples showed a high density of crabs, worms, and other invertebrates. After comparing bird counts, prey abundance, and tidal schedules, Li concluded that the availability of food—rather than shelter from predators or proximity to nesting sites—was the main factor influencing the birds’ choice of stopover locations.
Which choice best states the main idea of the text?
For SAT main-idea questions, read the entire passage but pay special attention to the first and last sentences, since they often frame and summarize the key point. Identify what the researcher or author concluded or argued, not just what they described. Then eliminate choices that (1) focus on a detail instead of the overall conclusion, (2) introduce new information not in the passage, or (3) contradict something the passage clearly states, and select the option that best matches the explicit conclusion or central claim.
Hints
Focus on the last sentence
Reread the final sentence that begins with "After comparing bird counts, prey abundance, and tidal schedules..." What does Li say he concluded at the end of that sentence?
Separate main point from details
Ask yourself: Is the passage mainly about tides themselves, the behavior of crabs and insects, the birds needing safety, or something else that Li identified after analyzing his data?
Watch for contrast words
Notice the phrase "rather than" in the last sentence. It shows what Li decided was not the main influence. What is being contrasted with those other factors?
Eliminate answers that contradict the conclusion
Check each choice against Li’s conclusion in the passage. Cross out any choice that focuses on a factor he specifically downplays or that the passage never actually mentions as a main result of his research.
Step-by-step Explanation
Identify what the passage is mainly describing
First, notice the overall focus of the passage: it describes ecologist Ramon Li’s expedition to coastal wetlands and his tracking of migratory shorebirds. All the details (mudflats, tides, crabs, worms, invertebrates) are part of his investigation into what influences where these birds choose to gather.
Locate the author’s explicit conclusion
Look closely at the last sentence, which starts with "After comparing bird counts, prey abundance, and tidal schedules, Li concluded that..." This is where the passage directly states the main takeaway from his research. The key part is the clause beginning with "Li concluded that" and ending with "was the main factor influencing the birds’ choice of stopover locations." That sentence tells you what his data showed was most important.
Distinguish main idea from rejected explanations
In that same conclusion sentence, Li contrasts one main factor with other possible factors. He says this main factor is the key influence "rather than" other things like shelter from predators or proximity to nesting sites. So, the passage is not mainly about tides alone, or shelter, or nesting sites; those are mentioned only to be ruled out as primary causes.
Match the stated conclusion to the correct choice
Now compare each option with Li’s stated conclusion. Only choice D, "Li’s observations indicate that food supply is the primary factor determining where migratory shorebirds gather," matches the passage’s final sentence: Li concluded that the availability of food, not shelter or nesting sites, is the main factor in where the birds stop.