Question 69·Hard·Inferences
Ecologists have long wondered whether city-dwelling birds adjust the timing of their dawn chorus to compensate for urban noise. Lena Wu et al. recorded the start times of the dawn songs of house sparrows at 30 urban sites during a week in early spring. They also measured traffic noise levels at each site for the hour after sunrise. At sites where average noise levels exceeded 70 decibels, sparrows began singing roughly 42 minutes before sunrise; at quieter sites (below 55 decibels) they began only 18 minutes before sunrise. The researchers then broadcast traffic noise at three of the quiet sites while keeping light levels unchanged; at these manipulated sites sparrows advanced their start times to match those at naturally noisy sites, beginning about 40 minutes before sunrise. Taken together, the results most strongly support the inference that ______
Which choice most logically completes the text?
For inference questions about experiments, first separate the observational pattern (what varies naturally across sites or groups) from the experimental manipulation (what the researchers deliberately change). Identify which variable was changed and which were held constant; then ask what behavior shifted in response. Use that cause-and-effect clue to eliminate answer choices that contradict the manipulation or introduce unmeasured factors. Prefer the option that directly matches the tested variable and observed outcome, without adding extra assumptions.
Hints
Locate the key parts of the study
Identify the two main parts of the research: the comparison among many sites with different noise levels, and the manipulation done at three quiet sites. What pattern appears in each part?
Pay attention to what the researchers changed
In the manipulated quiet sites, what exactly did the researchers add or change, and what did they deliberately keep the same? How did the sparrows’ behavior respond?
Test each answer against the data
For each answer choice, ask: Does the passage give direct evidence for this, or is it adding an idea that was never studied (such as specific causes the researchers did not measure)?
Step-by-step Explanation
Understand what the question is asking
The question asks what inference is most strongly supported by the results. That means we must:
- Summarize the key findings (both the observational data and the experimental manipulation).
- Choose the option that best matches what the data actually show, without adding new, unsupported ideas.
Summarize the observational result
First, look at the pattern across the 30 urban sites:
- Noisy sites (over 70 decibels): sparrows began singing about 42 minutes before sunrise.
- Quieter sites (below 55 decibels): sparrows began singing only 18 minutes before sunrise.
This shows a correlation: higher traffic noise is associated with earlier singing. But correlation alone does not prove what is causing what, so we need the experimental part.
Focus on the experimental manipulation
Next, the researchers created an experiment at three quiet sites:
- They broadcast traffic noise at these quiet sites.
- They kept light levels unchanged.
- After adding noise, sparrows at these sites began singing about 40 minutes before sunrise, similar to birds at naturally noisy sites.
Because they changed noise but did not change light, and the sparrows’ singing time shifted earlier, the data point to noise as the cause of the earlier singing, not light or random differences between sites.
Match the evidence to the answer choices
Now compare each option to the evidence:
- Any choice that blames light contradicts the experiment, because light stayed the same while behavior changed.
- Any choice that claims birds are responding to competitors from other sites is not supported; the study never measured that.
- Any choice that says timing is unaffected by noise or light ignores the clear change when noise was added.
The only choice that fits both the correlation (noisy sites sing earlier) and the experiment (adding just noise makes quiet-site birds sing earlier) is that traffic noise rather than light is the main factor causing urban sparrows to initiate dawn song earlier.