Singing over the noise
A For many songbirds, singing is not a pastime but a necessity. A male's song tells rivals that a territory is occupied and tells potential mates that a healthy, experienced bird is available. For the message to work, however, it has to be heard. Over many generations, the songs of each species have been shaped by the places where it lives: birds of dense forest, for example, tend to use sounds that travel well between trunks and leaves. Cities present these birds with an acoustic environment that, in evolutionary terms, appeared almost overnight.
B The most obvious feature of that environment is noise. Traffic, building work and machinery produce a constant low-pitched rumble, which overlaps with the lower notes of many birdsongs and makes them harder to pick out. Birds appear to respond in several ways. Some simply sing more loudly, much as people raise their voices in a crowded restaurant. Others shift their songs upwards, dropping their lowest notes or singing them at a slightly higher pitch, so that more of the song rises above the rumble.
C A third strategy is to change the timing. In some cities, birds begin their dawn chorus earlier than their relatives in the countryside, finishing much of their singing before the morning traffic builds up. Robins, which normally sing mainly by day, have been heard singing in the middle of the night beside busy roads. Explaining this behaviour is not straightforward, because the same places are often brightly lit, and artificial light may also persuade birds that it is time to sing. Separating the influence of light from that of noise remains a challenge for researchers.
D A further question is how these changes come about. Many songbirds learn their songs by listening to adults during their first months of life. A young bird in a noisy street may therefore copy the versions of a song that it can hear most clearly, which are likely to be the higher ones. In this way, a local 'city dialect' could develop without any change in the birds' genes, although some scientists suspect that natural selection plays a part as well. The adjustment may also come at a price. Lower notes can carry information about a singer's size and strength, so a bird that abandons them in order to be heard might, in theory, become less impressive to rivals and mates.
E What does this mean for city wildlife? The flexibility of some species is remarkable, but not all birds are able to adjust, and those that cannot may become scarce in noisy neighbourhoods. Observations made when traffic has fallen sharply for a few weeks suggest that birds can respond quickly when the noise drops. Members of the public can contribute by recording birdsong on their phones and sharing it with research projects, while planners can help by creating quieter green spaces, where thick vegetation softens some of the sound.