Ecologists have often attributed city-dwelling European robins’ earlier dawn song to artificial light at night: birds in brighter neighborhoods frequently begin singing sooner before sunrise. Mara Velez and colleagues, however, argue that traffic noise may drive much of this pattern. Because engine noise masks parts of robins’ songs, the birds could improve signal transmission by shifting more singing into the period before commuting begins. Ordinary urban-rural comparisons cannot easily test this account because municipalities with dense road networks also tend to install more nighttime lighting, whereas quiet sites tend to be darker. Thus, an association between brightness and early song could arise even if light has little direct effect on song timing.
Which finding, if true, would most directly support Velez and colleagues’ account?
For command-of-evidence questions involving competing explanations, identify the specific causal claim and any confounding variable mentioned in the passage. Then choose the finding that isolates the proposed cause while controlling for the alternative explanation; a simple correlation in which both factors change is weaker evidence.
Hints
Identify the competing explanations
Determine which two environmental factors could explain why urban robins begin singing earlier.
Look for a controlled comparison
Prefer evidence that varies one proposed cause while keeping the other approximately constant.
Check the quiet-site evidence
Consider what should happen at a brightly lit location if the environmental factor emphasized by the researchers is absent.
Step-by-step Explanation
Identify the researchers’ account
Velez and colleagues propose that robins begin singing earlier to avoid having their songs masked by morning traffic noise.
Account for the confounding variable
Because heavily trafficked places also tend to be brightly lit, a useful finding must separate traffic from illumination rather than merely compare typical urban and rural sites.
Evaluate the evidence
The first finding holds brightness approximately constant while showing that earlier traffic corresponds to earlier singing. It also shows that bright sites without early traffic do not have unusually early song, supporting traffic noise rather than light as the main explanation.