Cities are noisy places. If you ever get annoyed by the constant din of traffic, machinery and increasingly belligerent inhabitants, think about what songbirds must think. Many birds rely on songs to demarcate their territories and make their advances known to mates. They listen out not just for the sounds of seduction or rivalry, but for approaching predators and alarm calls that signify danger. Hearing these vital notes may be the different between life and death.
Last year, I wrote a feature for New Scientist about the effect that urban noise has on songbirds. Those that can’t make themselves heard are being pushed out of cities; others have developed strategies to rise above the clamour. British robins have avoided the traditional dawn chorus, when rush hour is at its peak, in favour of night-time singing when their tunes can stand out. German nightingales take the more straightforward approach of singing very loudly, belting out their songs at 95 decibels, enough to damage human hearing if sustained. And some species – great tits, house finches and blackbirds – have opted for higher notes, which are less easily masked by the typically low frequencies of urban noise.
So some species are adaptable enough to thrive in a cacophonous environment that would drive out those that can’t change their tune. And if the species that are driven away include predators and thieves, the birds that remain fare even better. That scenario is playing out in the cities of America. Clinton Francis from the University of Colorado at Boulder has found that noise reduces the diversity of bird communities but it actually helps those that remain.
Previous studies have linked the presence of noisy roads and industries with sparser populations of local birds, but never conclusively. Noise is also associated with habitat changes or visual disturbances, and it makes it harder for scientists themselves to spot birds – all of these factors could explain any disappearances.
To get around these problems, Francis relied on a unique natural experiment, taking place in the woodlands of New Mexico. Here, natural gas is pumped out of the ground and at some sites, it is then pushed along pipelines by compressors that are very noisy and that operate constantly. Other sites that lack compressors are much quieter but essentially the same in terms of environment and the surrounding trees. By comparing woods near noisy and quieter gas wells, Francis could isolate the effects of noise from those of the mere presence of industry. He even managed to get the compressors turned off for short windows while his team took stock of the local birdlife.
While the rapid expansion of human cities has been detrimental for most animals, some have found ways of exploiting these brave new worlds and learned to live with their prolific inhabitants. The Northern mockingbird is one such species. It’s very common in cities all over America’s east coast, where it frequently spends time around humans. But Douglas Levey from the University of Florida has found that its interactions with us are more complex than anyone would have guessed.
The mockingbird has the remarkable ability to tell the difference between individual humans, regardless of the clothes they wear. After less than a minute, they can tell one person from another and adjust their responses according to the threat they pose to its nest. This ability suggests that these birds are both intelligent and very flexible in their behaviour – two traits that must surely stand them in good stead in the urban jungle.
It obviously benefits an animal to be able to distinguish between threatening and harmless species, but discriminating between individuals of the same species is a much more difficult task – just think about how difficult you would find it to tell the difference between two mockingbirds by eye.
Levey worked with 24 pairs of mockingbirds that had taken up residence on the university’s campus. Hundreds of people walk within five metres of their nests every day and elicit absolutely no reaction. To simulate a greater threat, Levey asked one of his colleagues to approach the nests of birds with fresh clutches, and touch their rim for 15 seconds. When faced with such intrusion, mockingbirds will typically react by rallying from the nest, making alarm calls and diving aggressively at the trespasser.