Page 48 - Spring 2018

Anthropocene Soundscape Ecology
Figure 5. Flexible singing amplitude with fluctuating ambient-noise levels (based on Pytte et al., 2003; Brumm, 2004). Left: blue-throat- ed hummingbirds sing louder when their territorial song perches are noisier due to relative proximity to a noisy forest stream. Right: ur- ban nightingales in Berlin sing louder in territories that are noisier due to relative proximity to noisy street sections. Birds recorded on multiple days sing louder on noisy weekdays than on relatively quiet weekend mornings.
Anthropocene, human-altered landscapes have the poten- tial to have a big impact on animal communities. Some spe- cies disappear or decline, whereas others may enter a habitat and thrive. Consequently, avian communities in urban envi- ronments can still be relatively diverse. However, the species that inhabit urban communities are typically the same in cit- ies over large geographic ranges, whereas the species that oc- cur in the original and replaced natural habitats vary much more from place to place. Therefore, although local diversity may not decline, large-scale diversity does. This phenom- enon is known as urban homogenization (McKinney, 2002).
Acoustically, the Anthropocene is changing natural sound- scapes in several ways (Warren et al., 2006). Rachel Carson (1962) alluded to the potential of human pollution to detri- mentally affect the presence of singing birds in her influen- tial book Silent Spring. The turning of natural habitats into large-scale monocultures for agricultural crops and the use of pesticides indeed results in quieter mornings due to a de- cline in bird diversity and density. Although habitats have not become totally silent, changes in vocal animal commu- nities change the sounds that are filling the air. Dropouts and new arrivals change the competition and the patterns of acoustic interference for local communities (Slabbekoorn, 2017).
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Another prominent acoustic change of the Anthropocene is the emergence of increasing human activities that make loud and low-frequency sounds (Klump, 1996; Slabbekoorn and Peet, 2003). Road, rail, and air traffic is spreading in space and time and adds noise in large parts of the world. There is an almost continuous presence of man-made sound in urban areas and along many highways. Even in natural ar- eas and wildlife reserves, far-reaching sounds of traffic, pile driving, seismic surveys, or airplanes are more common. El- evated ambient-noise levels resulting from human activities have been shown to negatively affect birds in many ways, as discussed in Anthropogenic Noise Impact on Birds (Half- werk and Slabbekoorn, 2014).
Anthropogenic Noise Impact on Birds
The bird communities in noisy areas of otherwise suitable breeding habitats are less diverse and less dense compared with naturally quieter areas in equivalent habitats (Francis et al., 2009). Species that sing relatively low-frequency songs were more affected by the typically low-frequency sounds of generators or road traffic than were high-frequency sing- ers (reviewed in Slabbekoorn, 2013). More spectral over- lap between song and noise sources means more masking and more birds avoiding an area. Some species with high- pitched songs or calls may benefit indirectly from the pres- ence of low-frequency noise due to the negative effects on competitors with lower pitches, a phenomenon called com- petitive release. Breeding may be affected, along with migra- tion and survival, because birds foraging under relatively noisy conditions have a lower food intake rate (Francis and Barber, 2013). Foragers in noisy conditions are less efficient in pecking seeds from the ground because they need to look up more often to scan for potential predators that cannot be heard due to auditory masking.
There are also examples of male birds that persist in singing at noisy breeding sites but achieve a significantly lower pair- ing success compared with males at quieter sites (Halfwerk and Slabbekoorn, 2014). Potential mates in the noisy envi- ronment apparently do not hear the singing males or are not able to get sufficient detail from the song to evaluate their quality. It is also possible that females do not like to settle in a noisy territory and thus ignore the calling males. Finally, birds that do breed in noisier territories have significantly smaller clutch sizes and fledglings with lower body weights. This may be due to chronic stress causing reduced chick growth or to the masking of parent-offspring or parent-par- ent communication, leading to less efficient feeding patterns by the parents.