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  Figure 3. Spectral avoidance of ambient noise (based on Slabbekoorn and den Boer-Visser, 2006; Dingle et al., 2008). Left: South Ameri- can wood wrens sing high notes at high altitudes in the Andes where there is little competition from the local animal community for high frequencies. They refrain from using high notes at low altitudes where they compete with the sounds from a more diverse and dense animal community that is also more active due to temperature-dependent vo- cal activity. Right: great tits across Europe have been found singing consistently higher frequency songs in urban populations compared to great tits of nearby forest populations. This pattern has been replicated in this species in several studies across Eurasia up to Japan and for sev- eral other bird species that are common in both urban and forest areas.
community of invertebrates and amphibians who tend to be very active during large parts of the day in the lush and warmer habitat.
Flexibility for Fluctuating Conditions
Competition for acoustic space can be severe and may ren- der the effort put into signaling a waste of time and energy if songs and calls are largely or completely masked by other sounds. Consequently, several counterstrategies have been found in birds to enable them to cope flexibly in fluctuating noise levels. Some species can, for example, sing roughly at the same time of day, but avoid each other at a smaller tem- poral scale; one species may start to sing as another stops (Figure 4). Winter wrens are a particularly loud singing species that alternates singing with neighboring birds of the same species and also avoids conspecific songs when played back during an experiment (Yang et al., 2014). Tested indi- viduals shifted their timing by not starting a song during a playback song but initiated songs just after the playback. Another example of a noise-coping counterstrategy is found in blue-throated hummingbirds (Lampornis clemenciae), which sing louder when perched relatively close to noisy forest creeks or in response to playbacks of recorded stream noise (Pytte et al., 2003; Figure 5). This phenomenon is re-
Figure 4. Timing ability in singing birds (based on Yang and Slab- bekoorn 2014; Yang et al. 2014). Left: male winter wrens temporally avoid overlap with conspecific song (#1) by waiting for the other bird to finish a song before starting to sing themselves (#2). Right: winter wrens observed close to a road with fluctuating traffic noise levels and tested with experimental exposure were not found to avoid over- lap by using their timing ability (song #3 masked by traffic noise \[T\]). Together, these two findings have been interpreted to mean that over- lap avoidance is driven more by the need to hear competing sound than by the consequence of not being heard due to the competing sound (at least for this species).
ferred to as the Lombard effect and also occurs in people when ambient-sound levels rise, such as at a party (Brumm and Zollinger, 2011).
The habitat-dependent and natural noise-related varia- tion in frequency use between and within species can also result from a flexible response to current noise conditions (Patricelli and Blickley, 2006; Slabbekoorn, 2013). Because juvenile songbirds learn the songs of conspecifics, any mask- ing will impact adult song development; juveniles will not copy what they cannot hear and are therefore likely to end up with adult songs that are well-audible given local noise conditions. Similarly, immediate feedback may yield even more rapid adjustments. If birds get no response from oth- ers when using a song type that is heavily masked but they get a response from another, they may continue to repeat the latter. These kinds of signal adjustments are not evolution- ary changes, although the flexibility itself may be the result of an evolutionary adaptation.
The Anthropocene
Human settlement, the exploitation of natural resources, and the building of all sorts of infrastructure induce habitat destruction, degradation, habitat fragmentation, and a va- riety of noise pollution effects. As suggested in Living the
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