The paper on song sparrows studies the effect of the environment on the trill song of chipping sparrows, with a focus on divergent solutions to signal deterioration as a result of intraspecific song variation. First and foremost, sounds propagate through the air as waves composed of rarefaction and compaction zones. The density of air particles in the compaction zones is measured as the sound’s amplitude, and the distance between compaction zones is measured as the sound’s frequency. Sounds suffer from two kinds of degradation: frequency -dependent attenuation and reverberation. Attenuation is defined as a decrease in sound intensity with increasing distance. Ideally, sound spreads spherically from the source and intensity decreases in proportion to 1r2, where r is the radius of the sphere. However, environments tend to be heterogeneous, and habitat as well as weather conditions cause excess attenuation on top of the attenuation predicted by spherical spreading. In addition to attenuation, reverberation contributes strongly to impeding signal transmission. As sound waves impact objects they are reflected, which can lead to scattering, slurring of syllable form, and often makes it difficult to distinguish successive elements. These effects are most pronounced in signals of high frequency, high bandwidth, high duration, and low internote time. In order to compensate for attenuation and reverberation in highly obstructed environments such as forests, birds often decrease the minimum frequency and the number of amplitude and frequency modulations of their signals. This type of environmental tuning has been found within single species’ subpopulations living in environments with different levels of obstruction. Therefore, since signals are the primary means by which songbirds compete with one another, for individuals to be able to compete with one other, they must make their signals heard. Commonly, in order to transmit their signals above background noise, birds tend to both increase the amplitude of their vocalisations - known as the Lombard effect - and to sing higher pitched songs. Lastly, amplitude and frequency modulated trills are often used by songbird females as measures of vocal performance, based on how close the male sings to the biophysical limit of the bandwidth to rate ratio. Females prefer males which sing closer to the biophysical limit of trilling and males with stronger trills are better at holding their territories. In urban environments with highly reflective structures and high background noise, trills suffer heavily from the effects of reverberation and therefore vocal performance is perceived as being poorer. In turn, this has an effect on how well males can both attract females and defend their territories, which could mean that urban environments could generate unique intra- and intersexual selection pressures.
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