Several species of city birds have modified their songs in response to human-made noise
Dawn breaks in San Jose, the capital of Costa Rica. The city is still asleep, but early risers are greeted by a beautiful symphony: hummingbirds, corn eaters, yigüirros (clay-colored thrushes), yellow-breasted hawfinches, blue tanagers, wrens, warblers and other birds herald a new day.
Soon, the incessant din of vehicles and their horns, construction, street vendors and more takes over, forming the soundscape of the frenetic routine of millions upon thousands of people who travel and live in this city. Then the birdsong fades into the background.
“Bird song has two main functions in males: to attract females and to defend their territory from other males,” says Luis Andrés Sandoval Vargas, an ornithologist at the University of Costa Rica. For females in the tropics, he adds, the primary role of their song is to defend their territory. So, in order to communicate, keep their territories safe and find mates in cities, birds need to find ways to counteract the effects of anthropogenic noise – that is, the noise made by people.
“The main effect of urban development on song is that many birds sing at higher frequencies,” says Sandoval Vargas. For example, studies over the past 15 years have found that blackbirds (Turdus Merula), great tits ( Released major) and Red-collared Sparrows ( Zonotrichia capensis) sing at higher pitches and higher minimum frequencies in urban settings than in the countryside.
But the birds’ response to anthropogenic noise may be more complex, as Sandoval Vargas found when studying wrens (wren aedon). Wrens are small, brown birds — about 10 centimeters tall and 12 grams — that feed on insects and tend to live near humans. In Costa Rica they can be found almost everywhere, but they are particularly numerous in the cities. “Males sing almost year-round and sing for many hours during the day, and much of their behavior is mediated by vocalizations,” explains Sandoval Vargas. But what makes them ideal for studying adaptations to urban environments is that most components of their song are in the same frequency range as the noise we humans make.
Over two years, taking advantage of the house wren breeding season—April to June—at four locations in Costa Rica, Sandoval Vargas and his team recorded the song of male house wrens and also recorded ambient noise. Although all four sites are in urban areas, man-made noise levels are different at each site, ranging from very high and medium-high to medium-low and low.
The study, published in 2020 in International Journal of Ornithologyfocused on the repertoire of sound elements – the variety of unique sounds that, when combined together, form a bird’s distinctive song – produced by the wrens.
As the scientists expected, wrens tended to sing at higher pitches in places with anthropogenic noise. But that’s not all they discovered.
They also found that, in general, the size of the birds’ repertoires decreased with increasing anthropogenic noise, particularly when the birds were exposed to anthropogenic noise levels above the usual noise to which they were accustomed. The researchers observed the same pattern at the individual level: the same bird presented a lower song repertoire on louder days than on less noisy days.
A reduced repertoire can affect how well these birds learn their phonetic language, as songbirds need to hear themselves and other birds to crystallize their song. “What’s happening here is that they lose some of their vocabulary and some of their sounds because they don’t produce them. And in these species, juveniles need to listen to adults to learn to sing,” says Sandoval Vargas.
In the long term, this could make it difficult for the birds to communicate with other populations of their species. Suppose you had a situation where you have a large population and a small population, and to maintain the small population you wanted to move individuals from one to the other, explains Sandoval Vargas. “But it turns out that the individuals of the small population in the city sing very differently than those of the large population … they will not recognize them. And because they can’t communicate, they can’t reproduce either [with them]’ says Sandoval Vargas.
Over time, this could induce the onset of speciation processes – that is, individuals in the city develop differently than those living in rural habitats.
Birds resort to a variety of strategies in the face of human noise. canaries (Serinus Serinus) — common birds in Spanish cities — sing longer when there’s more noise in the city to offset that noise, says Mario Díaz Esteban, a researcher at Spain’s National Museum of Natural Sciences who led the research that came to this conclusion in 2011.
However, this tactic has its drawbacks. “The problem is that if a person has to spend a lot of time singing to compensate for the noise, that time can’t be used for other functions like foraging, mate-finding and, probably most importantly, predator-watching.” explains Diaz Esteban.
The price of living in the city
Changes in the songs of canaries and house wrens are an indication that the birds, like many other creatures, are slowly – and in different ways – adapting to thrive in urban environments.
Australian ecologists Mark McDonnell and Amy Hahs noted in a 2015 article Annual review of ecology, evolution and systematicsthat organisms that can change their phenotype—observable traits such as body shape, development, or behavior—in response to environmental conditions are more likely to survive in changing environments and adapt to new conditions.
Changes in song are just one of many adaptations that birds exhibit when living in cities. They can also fly slower. “There are a lot of humans moving around in urban environments, and birds can perceive this as some level of risk or threat…when a human approaches, they have a distance they will tolerate before taking off,” explains Hahs von from the University of Melbourne. The same could happen around pets or vehicles, she adds.
City birds also changed their diet. Hahs refers to the classic example of European blue tits (Cyanistes caeruleus), which learned to steal milk by opening bottles while normally feeding on insects. “The big example in Australia is the ibis, which normally feed in wetlands but have started stealing waste from the bin,” she adds.
Díaz Esteban says that while the impact of human activity on birds in general can be negative, there may also be species “that benefit from human proximity, either because there is more food, fewer predators, or their competitors are less tolerant of human presence.” But, he says, there’s not much evidence that song modification is such an advantage for birds in urban settings.
And although the presence of humans and the construction of cities put pressure on bird behavior, there are many avenues of protection within cities, say McDonnell and Hahs. They add that there is an urgent need to identify measures to create biodiversity-friendly cities.
“If we’re able to reduce some of the urban impacts in our cities – create more green spaces, reduce urban heat islands through vegetation and other measures.” [such as] Finding ways to make habitats more connected,” says Hahs, “then more species that live in our cities will find the urban environment less of a challenge.”
Article translated by Debbie Ponchner
Leah in Spanish
This article originally appeared in Interesting Magazine, an independent journalistic endeavor by Annual Reviews. Sign up for the newsletter.
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