Multiple Choice, Single Answer

Cities are noisy places for animals that depend on sound to communicate. Traffic produces most of its noise at low frequencies, and this background rumble can mask the songs of birds that sing in the same range. Studies in several European cities have found that great tits living near busy roads sing at a higher minimum pitch than those in quiet forests, and that some species sing earlier in the morning, before traffic builds up, or even at night. At first, researchers assumed that these changes were simply useful adjustments that allowed urban birds to be heard. More recent work suggests the picture is less straightforward. In some experiments, female birds responded more strongly to lower-pitched songs, which are often associated with larger, healthier males. A male that shifts to a higher pitch to be heard over traffic may therefore become easier to hear but less attractive. In addition, singing louder or for longer has an energy cost, and birds that are kept awake at night by artificial light and noise may get less rest. Whether these birds are adapting successfully or merely coping is still debated. Answering the question will require long-term studies that measure not just how birds sing, but how many young they raise in noisy and quiet areas.