Several species of birds that live in cities have changed their song in response to human-generated noise
“Birdsong has two main functions in males: to attract females and to defend the territory from other males,” explains Luis Andrés Sandoval Vargas, an ornithologist at the University of Costa Rica. For females in the tropics, he adds, the main role of chant is to defend the territory. So, to communicate in cities, to keep their territory safe, and to find mates, birds must find ways to counteract the effects of anthropogenic noise, i.e. human-made noise.
“The main effect of urban development on song is that many birds sing at higher frequencies,” explains Sandoval Vargas. Studies conducted over the past 15 years have found, for example, that blackbirds ( Turdus merula ), great tits ( Parus major ), and rusty-necked sparrows ( Zonotrichia capensis ) sing at higher frequencies, with higher minimum frequencies in urban environments than in rural ones.
But the response of birds to anthropogenic noise may be more complex, as Vargas discovered by studying wrens ( Troglodytes aedon ), birds of the passerine family. Wrens are small brown birds – about 10 centimeters tall and 12 grams heavy – that feed on insects and tend to live close to humans. In Costa Rica they are especially abundant in cities. “Males sing almost all year round and sing for many hours during the day, and much of their behavior is mediated by vocalizations,” explains the researcher. But what makes them ideal for studying adaptations to urban environments is that most of the components of their song fall within the same frequency range as the noise produced by us humans.
For two years, taking advantage of the breeding season of house wrens – from April to June – Sandoval Vargas recorded the song of male house wrens in four locations in Costa Rica, also recording ambient noise. Although all four sites are located within urban areas, human-generated noise levels are different at each site, ranging from very high and medium-high to medium-low and low. As the scientists expected, the wrens tended to sing higher-pitched in places with more human noise. But that’s not all they discovered.
The loss of the singing repertoire
They also found that, in general, the size of the birds’ song repertoire decreased with increasing anthropogenic noise, especially when the birds were exposed to noise levels higher than the noise they were used to. The researchers observed the same pattern at the individual level: the same bird offered a smaller song repertoire at noisier sites than at quieter sites. A small repertoire can affect these birds’ ability to learn their aural language, as songbirds need to hear themselves and other birds to crystallize their song. “What is happening is that they are losingpart of their vocabulary and some of their sounds, because they don’t produce them. In these species, the juveniles need to listen to the adults to learn to sing,” explains Sandoval Vargas.
While human presence and the construction of cities put pressure on bird behaviour, there are also many conservation opportunities within cities, and actions to create biodiversity-friendly cities are urgently needed. “If we can reduce some of the urban impacts in our cities – by creating more green space, then more species in our cities will find the urban environment less challenging,” say other co-authors of the study.
- House Wrens Troglodytes aedon reduce repertoire size and change song element frequencies in response to anthropogenic noise. International Journal of Avian Science (onlinelibrary.wiley.com)