You can hear a great tit or house sparrow singing away from a lamppost or a scraggly street tree at seven in the morning if you’re standing close to a busy intersection in almost any major city. Its voice is noticeably higher than you’d expect from the same bird in the countryside. It’s simple to pass by without realizing it. But it’s difficult to unhear once you know to listen for it.
This trend has been documented for years by researchers, and when you think about it, the explanation is quite simple. The low-frequency range is primarily occupied by city noise, which is the low rumbling hum of machinery and traffic. Nearly the same range is occupied by bird songs, particularly the older, deeper notes that certain species evolved for forest or open terrain. The birdsong loses when the two collide. As a result, the birds have begun to adapt, raising their songs to frequencies that are inaccessible to the vehicles.
There’s something almost praiseworthy about this fairly exquisite bit of development taking place in real time. Compared to their rural cousins, great tits examined in European cities sing songs that are substantially higher in pitch, and this tendency persists in a variety of urban settings. Similar behavior is displayed by song sparrows in several regions of North America. Researchers believe that this isn’t a single species acting strangely, but rather a widespread reaction that manifests itself wherever traffic noise and birds coexist.
What gets forgotten in the process is the problem. In many species, deep notes convey specific information, such as a resonance that indicates a bird’s size, fitness, or even characteristics that females use to determine whether a male is a suitable mate. When you remove those bass notes, the result is something that is technically audible but somewhat thinner. Although a few field studies indicate that higher-pitched, simpler urban songs are associated with reduced attraction rates in some species, it is still unknown how much this truly costs birds in terms of mating success. Not disastrous. However, it’s also not nothing.
Although it receives less attention than the mating angle, there is also a territorial cost. High-pitched notes don’t go as far as low-pitched ones, and warning calls and boundary songs rely in part on the speed of sound. It’s possible that nearby men who depend on those signals to avoid getting tangled with one another won’t receive the warning in time. Boundaries become hazy. It’s unclear if this results in more real bird-to-bird violence or simply more energy being squandered due to unclear territorial conflicts.
The generational aspect of this is possibly the most remarkable. Just like children pick up an accent from those around them, young birds learn their songs by listening to adults. The species’ song is handed down if the adults are already singing a simplified, high-pitched, noise-adapted version of it. After a few generations, the old, richer version of the music might simply cease to exist in cities entirely, to be replaced by a louder, leaner version designed just for traffic and concrete.
It’s tempting to describe this as a minor tragedy as it develops, and perhaps it is one that occurs subtly and on a scale that no one is aware of on a daily basis. However, it also demonstrates that birds adapt more quickly than most people realize. These studies don’t actually address the much more difficult question of whether that flexibility is sufficient to counteract everything else cities throw at wildlife, such as habitat loss, light pollution, and glass structures.
