City birds and the noise problem
Cities are noisy places. Traffic, building work and crowds of people fill the air with sound all day long. For birds, which use song to attract partners and to defend their territory, this noise is a serious problem: a message that nobody hears is useless.
Scientists who have studied birds in several European cities noticed that some species have changed the way they sing. In noisy streets, great tits often sing at a higher pitch than their relatives in quiet forests. City noise consists mostly of low sounds, and high notes are less likely to be covered by the deep rumble of engines. Other birds simply sing louder, or begin their songs earlier in the morning, before the rush hour starts.
However, these changes have a price. Singing loudly costs energy, and a bird that uses an unusual pitch may be misunderstood by others of its kind. Researchers therefore suggest that cities should keep quiet green areas, such as parks with thick bushes, where birds can still communicate in a natural way.