The paper on song sparrows studies the effects of the transfer function, reverberation, and noise masking of the environment on the trill song of chipping sparrows, with a focus on divergent solutions as a result of intraspecific song variation. A transfer function is a way of describing how the environment filters sound and causes frequency pattern degradation. Urban environments often have transfer functions which favour an intermediate frequency range above low frequency background noise but below frequencies that easily reverberate. Reverberation is a temporal pattern distortion that contributes strongly to impeding signal transmission by adding a tail to notes. As sound waves impact objects they are reflected, which can lead to slurring of both syllable form and pattern. Additionally, due to reverberation, sounds take different paths that can cause both amplitude and frequency interference. The effects of reverb are most pronounced in signals of high frequency, high bandwidth, high duration, and low internote time. In order to compensate for reverberation high obstruction environments such as cities, birds often change their signals by: decreasing minimum frequency, decreasing the number of amplitude and frequency modulations, and increasing internote time. However, in order to transmit their signals above background noise, birds tend to increase the frequency and amplitude of their vocalisations. Although, these modulations can often be hard to separate due to the Lombard effect. Lastly, amplitude and frequency modulated trills are often used by female songbirds as measures of vocal performance, based on how close the male sings to the biophysical limit of the bandwidth to rate ratio. Males which sing closer to this limit are preferred by females and can better defend 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 means that urban environments could generate unique intra- and intersexual selection pressures.
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