Cities are getting louder. San Francisco, in particular, has gotten about six decibels louder since the 1970s — a significant amount considering adding 10 decibels makes something seem twice as loud. The added noise, called anthropogenic sound (meaning caused by humans), mostly comes from low-frequency car traffic sounds, but also includes other regular city noises, such as air traffic, construction, sirens and car alarms. This is bad news for the little birds that rely on song to attract mates and defend their territory. In loud city centers, birds now have to fight to be heard.
David Luther, a biology professor at George Mason University, says various species have begun to sing higher notes to be audible over urban din. “A response in many different taxa has been an increase in the minimum frequency of acoustic signals, which increases signal transmission and detectability by reducing overlap with low-frequency anthropogenic noise,” Luther says. But for the birds, this has a negative effect.
“You can tell a lot about a bird by how it sings, whether it’s trying to attract a mate or defend its territory. Because many noises are physically difficult for birds to produce, you can glean information about the health of a bird from its calls,” Luther explains. But it’s not just the scientists gleaning the information. Potential mates or nemeses also gauge each other’s fitness by each other’s songs. Trills, the repetition of a syllable, require precisely coordinated vocal tract movements. They impress each other with fast trills in a broad bandwidth of sounds, since a variation of notes requires they open their beaks widely. “It’s like clapping,” Luther says. “If you open your hands wide you can’t clap as fast.” The ability to produce this tradeoff is called “vocal performance.”
In his recent paper, “Not so sexy in the city: urban birds adjust songs to noise but compromise vocal performance,” Luther and his colleagues compared the evolving songs of white-crowned sparrows in the Presidio of San Francisco and their rural cousins 40 miles north in Marin County to see how the birds compensate in the increasingly deafening city. They found that in louder areas, the birds indeed try to sing at a higher pitch to cut through the low-pitch noise, which in turn decreases the range of notes they sing, hurting their vocal performance.
They also found something they didn’t expect: The birds were singing faster trills to compensate for their lack of bandwidth. “We didn’t know the extent to which the background noise would change the rate,” Luther says. Sadly, though, the faster trills aren’t enough. By gauging the reactions of the other male birds, the researchers found that city birds have low-performing songs.
So what’s going to happen as cities keep getting louder? Will the males just keep singing higher? “I don’t think they can,” Luther says, “because they have to stay within the hearing range of the other birds.” Luther predicts that if they continue to exist in cities, which “are not easy environments for wild animals,” they’ll evolve to have specifically different traits — mainly the pitch ranges of their songs and the mating preferences of the females — from birds of the same species that don’t live in cities.
Luther and some colleagues are currently indirectly testing this hypothesis by looking more closely at the lady birds’ preferences. “We’ve done a number of studies about how males respond to the differences in songs, but little has been done with females and what they prefer, who they prefer to mate with,” Luther says. They collected baby female birds and are hand-rearing them in a lab. This spring, when the birds are mature, they will give the birds extra estrogen, play them different songs, and count their “copulation solicitation displays.” Or, they might plant two speakers playing different songs near the same bird and see which speaker the bird spends more time cozied up to. (It’s fun to imagine what the equivalent experiment would be were aliens conducting it on humans.)
In a strange twist, anthropogenic noise, a type of pollution, could end up adding to biodiversity. Luther believes that if female preferences do indeed change, the species would split into two species. The rural birds will presumably maintain their lower frequencies no longer tenable for their urban counterparts, and the urban female birds will, well, permanently lower their standards, eventually becoming a different breed of trilling, high-pitched birds.
The Science of Cities column is made possible with the support of the John D. and Catherine T. MacArthur Foundation.