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 lection over an evolutionary time scale, will emerge. Unfor- tunately, this picture is being challenged by noise pollution, which is emerging over a much more rapid ecological time scale (Slabbekoorn and Ripmeester, 2008).
I start by addressing how animals have evolved in the con- text of their environment and how they are perceived by other animals who can affect their survival and reproduc- tive success. This sets the stage for understanding how birds deal with the natural soundscape in which they live, through evolution and natural selection and through learning from experience and adjusting their behaviors accordingly. This clarifies the intricate acoustic relationship animals have with their environment that is now being modified in the indus- trializing and urbanizing world.
From Camouflage to
Perceptual Contrast
The snow-white fur of an arctic hare (Lepus arcticus) makes it hard to find in a snow-covered landscape, but it is easily detected in a green meadow (Figure 1). This is true in gen- eral: if the spectrum of an object matches its background, it is well camouflaged, and if not, it stands out. Patterns of foreground-background matching have often evolved over time through natural selection when it is advantageous to not be seen, in both predators and prey. However, if it is ben- eficial for an animal to be seen, the opposite can be expected. For example, if transmitting a signal to conspecifics deter- mines success in mate attraction, it is better to have contrast- ing rather than matching color spectra. The bright plumage and skin of many birds and frogs often stand out dramati- cally against their typically green and brown surroundings for this reason.
These phenomena of camouflage and perceptual contrast apply to audition as well as to vision (Figure 1). Sounds can blend in or stand out against the ambient background simi- lar to visual objects (Handel, 1989; Endler, 1993). Acoustic events can match the ambient background “noise” in spec- tra, amplitude, and temporal patterns, thereby going un- noticed. In contrast, acoustic events can also have features that increase the perceptual salience of sounds relative to the ambient conditions, such as different frequencies or timing, high signal-to-noise ratios, and distinct temporal patterns.
Whether we find patterns of camouflage or contrast in the acoustic mode of the natural world can usually be explained by natural selection (Brumm and Slabbekoorn, 2005). Sounds that are not produced with a purpose (e.g., just as a
Figure 1. Top: visual camouflage and contrast illustrated with a snow hare in a snow-white landscape and against a green background after snow melt. Bottom: perceptual salience of sounds (see Handel, 1989; Endler, 1993) explained in two schematic spectrograms. Left: exam- ples of how signals can blend into their acoustic surroundings. Right: signals that stand out against the ambient soundscape. Stars indicate how spectral overlap or deviation between signal and ambient noise may cause or avoid masking; diamonds reflect how timing can af- fect masking; triangles illustrate how similarity in shape and pattern make it hard to detect the presence of target sounds among nontarget sounds. The distinct repetitive pattern of the flattened squares of two different frequencies shows how sounds can also stand out against that background of triangles.
by-product of moving around) could become cues to third parties such as potential predators and prey. Consequently, such sounds have often become soft and low contrast so as not to be detected. Examples include the rustling sounds of potential prey insects and birds. Another example is hunting owls, which have evolved wing morphologies and flight skills that are very silent as the owls attack prey. If being heard has an advantage, evolution can take another direction; doves taking flight on alarm or snipes performing aerial acrobatics in nocturnal displays have evolved wing beats and special feathers to generate sounds during flight with their wings.
Some sounds (e.g., begging and contact calls) are usually relatively faint because they are only meant to be heard by a few nearby receivers. If the sounds were louder, they could become cues to third parties with negative consequences for the vocal animal. However, there are also examples of loud and raucous calls as well as vocal courtship interactions of high amplitude and intensity, which shows that natural se- lection pressures may vary across species and context. The specific evolutionary outcome is often the result of a trade- off between the costs and benefits of being detected, local- ized, and evaluated as worthwhile by different animals look- ing for prey or partners (Read et al., 2014).
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