Page 49 - Spring 2018
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Another effect of man-made sounds on birds that has been reported for many common urban areas all over the world is vocal adjustment. The flexible song features that are found in natural settings with fluctuating noise levels apparently also allow many species to adjust to noisy human habits in the city. Urban great tits (Parus major) use more high-fre- quency song types in the presence of low-frequency traffic noise (Figure 3), in comparisons both among individuals within a single population (Slabbekoorn and Peet, 2003) and among populations in urban and forested areas across Eu- rope (Slabbekoorn and den Boer-Visser, 2006). Playback ex- periments suggest that the spectral shifts in urban great tits have the potential to be adaptive. Male great tit communica- tion toward females is negatively affected by the overlapping spectrum of traffic noise in the case of low-frequency songs but not for high-frequency songs (Halfwerk et al., 2011).
Coping Flexibly in Urban Settings
The level-dependent use of different frequencies in great tit songs can be explained by relatively fast masking-dependent switches between low- and high-frequency song types with- in individual repertoires (Halfwerk and Slabbekoorn, 2009). If great tits sing low-frequency songs and the ambient-noise level is experimentally elevated by playing back an urban noise spectrum, the birds switch more quickly to another song type than if they are singing a high-frequency song type. Interestingly, a recent repeat of the experimental expo- sure test in black-capped chickadees (Poecile atricapillus) re- vealed that birds may have to learn to cope in such a flexible way; an upward shift in song frequency use on the elevating ambient-noise level depended on whether local territories were more or less urban and thus whether territorial birds had more or less experience with conditions of fluctuating levels of traffic (LaZerte et al., 2016).
At airports, birds can be exposed intermittently to very high levels of broadband noise from which there is no spectral es- cape. In such cases, singing birds stop and temporally avoid the periods of noisy takeoff and nearby overflights (Dominoni et al., 2016). The fluctuation in ambient-noise level at the scale of minutes is also typical for the heterogeneous cacophony of busy traffic on city roads along which birds may breed in ur- ban shrubs and trees. Winter wrens, for example, are often found defending their territories acoustically at places that can be very noisy when cars and trucks are passing by. Al- though I previously discussed in Flexibility for Fluctuating Conditions the capability of timing song production neatly into relatively quiet slots of neighboring conspecifics, males of this species did not temporally avoid masking (Figure 4), ei-
ther from actual traffic noise or from an experimental broad- band and intermittent noise exposure (Yang et al., 2014). Ap- parently, the task is either too difficult or not beneficial in this particularly loud-singing bird species.
Finally, urban nightingales in Berlin, Germany, sing louder in noisy than in quiet territories (Figure 5), and a few males that were recorded at multiple occasions were reported to sing louder during noisy weekdays than during the quieter weekends (Brumm, 2004). These urban birds thus seem to respond in the same way as hummingbirds at noisy rain- forest streams and humans at noisy cocktail parties: raise your voice to cope with noise. Interestingly, for some bird species, it has also been found that singing louder is inher- ently related to singing at higher frequencies (Brumm and Zollinger, 2011). This increase in frequency has likely to do with the mechanistic challenges of producing sound at both low frequencies and high amplitudes. The human voice also raises in pitch when shouting or calling to reach someone at a distance. This implies two changes in acoustic design that could benefit auditory perception under noisy urban condi- tions: improvement of the signal-to-noise ratio by increas- ing volume and spectral avoidance.
In conclusion, the diversity in the animal kingdom is re- flected in the diversity of acoustic communication systems, which is shaped by diversity in the environment. Species- specific evolution of acoustic repertoires allows each species to stand out when needed for mate attraction or deterrence of competitors and to blend in when predators are among the potential eavesdroppers. The species-specific acoustic design or elaboration of signals is driven by sexual and natu- ral selection, given the sensory performance of receivers of sounds that are filtered during sound propagation and af- fected by noise interference in their habitat. This phenom- enon is referred to by evolutionary ecologists as “sensory drive,” although “environmental drive” may be a better term because it is the environment that determines the direction of the drive.
Acoustic diversity and evolutionary shaping is particularly true and well studied for birds. However, other taxa using acoustic signals are affected by the same aspects of environ- mental selection (Francis and Barber, 2013; Wiley, 2015; Slabbekoorn, 2017). Some frog species have evolved the use of ultrasonic frequencies when living close to fast-flowing streams that produce low-frequency noise. Many fish spe- cies produce sounds that propagate well in the aquatic envi-
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