Urban areas are noisy. The din of a plane taking off at the local airport, the honk of car horns and the hubbub of people in the streets are examples of sounds characteristic of big cities. This constant background noise, and the presence of tall, sound-reflecting buildings, have forced city birds to sing louder and clearer to make themselves heard. But more than simply learning to adapt to city life, it seems urban birds may be evolving to become a new species, one with vocal traits that ensure survival in noisy environments.
There is little doubt that singing habits of urban birds differ from those of country birds; for example, European robins living in cities sing only at times when urban noises are reduced. But the reasons for these differences are not clear. Do city birds learn to sing louder as they mature? Or is natural selection in action in which case birds with certain vocal abilities imprinted in their genes are the ones surviving?
To try to understand the relative influences of adaptation and evolutionary changes, scientists at the University of Melbourne studied acoustic communications of silvereyes, songbirds abundant in southern Australia. Typically, their sounds have frequencies between 2 and 6 kilohertz so they are mostly masked by urban noises, which have frequencies in the 1 to 4-kilohertz range.
In a paper published last month, researchers explain that they recorded 81 male birds in various Australian cities and compared their sounds with those recorded in corresponding rural areas. For example, the sounds of Melbourne silvereyes were measured against those recorded in the Lerderderg State Park, an hour’s drive from the city.
Both songs and contact calls were studied. Calls are typically shorter and simpler than songs and are used to ask for food, in danger situations or when a bird is trying to locate other members of its flock. Most importantly, while songs can be learnt, contact calls are innate and remain mostly unchanged throughout a bird’s lifetime.
“Thus,” researchers write, “if calls as well as songs display predictable variation in relation to urban noise, this suggests that such responses result from evolutionary changes, rather than phenotypic plasticity” (the learning of new skills to adjust to changed settings).
The study found that city birds express themselves at higher frequencies than those living in rural areas. Significantly, scientists found changes in both songs and calls with the former being on average 195Hz higher in cities and the latter 90Hz higher.
In addition to singing louder, researchers found that urban silvereyes sing fewer syllables per second, effectively making their songs slower. Given that tall buildings reflect sound, which results in songs less clear, researchers believe that birds are making their songs slower so that they are easier to understand if the sound is distorted by human constructions.
Because songs and calls are essential to the birds’ communication and to ensure mating success, silvereyes that don’t adjust to city living by changing their songs and calls may be unable to breed. Therefore, the authors of the study gather that cities are pushing birds to evolve by selecting for certain genetic changes related to specific vocal abilities.
“Urban birds might be becoming genetically distinct,” Dominique Potvin, the lead author, said to New Scientist, “which is the first step towards becoming a new ‘urban’ species.”