Figure 1.

Illustration of our prediction that urban birds are more likely to be broadly tolerant of environmental conditions than rural congeners. Fitness of a bird with narrow tolerance (solid curve) drops off steeply with any changes in the environment. In contrast, a bird with broad tolerance (dotted curve) has maximum fitness across a wide range of environmental conditions, which drops off at more extreme conditions. If an area where the two species coexist becomes urbanized (arrow), environmental conditions (vertical lines) will shift, and are more likely to fall within the range of tolerance of a bird with broad environmental tolerance than that of a bird with narrow tolerance.