Goldfinches living by the Madrid airport no longer sing at dawn. Instead, they start chirping before the first flights of the morning start to drown them out. It's an extreme example of something that's happening in every city: As cities get louder, wildlife are changing to adapt to the noise.
A recent study looked at sparrows living in San Francisco, where noise levels have gone up by six decibels since the 1970s. In response, sparrows now sing higher notes to stand out from the low roar of traffic. "[Their trill] gets faster, more notes closer together, and the lowest frequency gets higher when it is louder," says David Luther, a biologist at George Mason University and author of the study.
As the song changes, female birds are less impressed. "A male with higher performance will receive more mating opportunities," says Luther, explaining that as the birds strain for high-pitched notes, the range they can sing shrinks, making for a lackluster show. The study found that the shrinking bandwidth of notes is likely the reason that rival male birds are also unimpressed by city songs.
A few miles away, over the Golden Gate Bridge in Marin County, rural sparrows still sing lower-pitched trills. If the city gets even noisier in the future, it's possible that the two populations of birds might eventually evolve into two species, with a new species of city birds that has females with lower standards.
But it's also possible that the city could go in the other direction—if, for example, self-driving electric cars become the norm and gas vehicles eventually disappear, helping reduce one big part of urban noise. And if that did happen, sparrows might go back to their old songs. "It's possible that they could revert to lower frequency songs, like they had 40 years ago," Luther says.