Question 78·200 Super-Hard SAT Reading Questions·Information and Ideas
Citywide surveys often report that birds begin nesting earlier in cities than in nearby rural areas, a pattern commonly attributed to warmer urban temperatures. An analysis of 20 years of nest records found that cities contained a much larger proportion of year-round resident species, which typically nest earlier than migratory species in both habitats. The average date across all city nests was indeed earlier than the average across all rural nests. But among each of 18 species well represented in both habitats, the median nesting dates in the city and countryside differed by no more than one day. Moreover, unusually warm springs advanced nesting by similar amounts for the urban and rural populations of a given species. Taken together, the analysis suggests that ______
Which choice most logically completes the text?
For inference questions involving averages, check whether the groups being compared have different compositions. Then compare the overall pattern with evidence about matched subgroups and avoid choices that confuse a year-to-year response with a difference between locations or populations.
Hints
Compare levels of analysis
Distinguish the comparison involving all city and rural nests from the comparisons involving the same species in both habitats.
Consider composition
Ask how the different proportions of resident and migratory species could affect each habitat’s overall average.
Interpret the temperature evidence
Consider what the similar responses to unusually warm springs show—and what they do not show—about the difference between cities and rural areas.
Step-by-step Explanation
Compare the overall averages
Across all nests, the city average is earlier than the rural average. Considered alone, this result might appear to support an effect of warmer urban temperatures.
Examine species composition
Cities contain a greater proportion of resident species, and resident species generally nest earlier than migratory species in both habitats. Thus, the two overall averages are based on different mixtures of species.
Use the within-species evidence
For every species represented well in both habitats, urban and rural nesting dates differ by no more than one day. This indicates that the large difference between the overall averages is mainly a result of species composition rather than a substantial habitat-specific change in each species’ schedule.
Account for warm springs
Both urban and rural populations advance nesting by similar amounts during unusually warm springs. Temperature can therefore affect nesting time without producing the observed city-specific difference in the overall averages.