Listening to bird songs in nature helps relieve our work stress and raise productivity. On the other hand, while we enjoy the sounds of nature, are we aware that the noise we generate is affecting the survival of the birds living next to us? Birds rely on vocal signals to communicate, guard territory, locate position and court their mates. The noisy urban environment is causing problems for them. In response, many birds are shifting their voices. Have you noticed the subtle changes?
In the city, the ambient environment is harsh for birds due to a masking effect by all the crowds of people, vehicles and buildings that can smother sounds. Effective transmission of the birds’ calls seems difficult if not impossible! Imagine what it is like to communicate with a friend some distance away on a noisy street. Studies have indicated that urban noise suppresses the number of eggs laid by birds and the population size. To cope with such adversity, many birds have developed special vocal adjustment techniques.
The most direct response to a noisy environment may be to raise the volume. However, calling in such a way highly depletes one’s energy and few birds will go for such a “costly” strategy. Scientists have noted that some birds adopt shorter calls or repeat certain notes to accentuate their voices. Others may even change the timing of calls, to avoid "sound combat" with the ambient environment.
Most birds, however, shift their tune frequencies. In general, city noise belongs to the low-frequency range from about 1000 to 2000 Hz. A higher pitch can be heard against such background noise.
By altering the head angle and beak gape the birds can shift their calling tunes. Less extra energy is needed. Research on local birds concluded that many raise their pitches in a noisy environment, including Common Tailorbird Orthotomus sutorius, Fork-tailed Sunbird Aethopyga christinae, Oriental Magpie RobinCopsychus saularis, and Spotted Dove Spilopelia chinensis. In the bird world, having a high-frequency voice is not only to show off but essential for their survival!
Nevertheless, pitch raising has its disadvantages too. Higher pitch propagates a shorter distance in the air. There is also a morphological limitation in changing pitch. Birds having a narrow range of vocalisation cannot take advantage of the vocal adjustment competition.
"Retreat" in the City
One of the major objectives set out by the Environment Bureau in The Hong Kong Biodiversity Strategy and Action Plan 2016-2021 is to raise biodiversity in the city. To create quality bird habitats in the urban area, noise reduction is a priority which can be achieved by proper city design. The Tang-styled Nan Lian Garden in Wong Tai Sin can be a reference. A well-designed sound barrier has been adopted so that visitors are not affected or even unaware of the fact that the garden is surrounded by roads and traffic. It is indeed a nice retreat for birds as well as people in the city.