Summary: Cardoso & Atwell 2011

Some oscines (songbirds) shift to singing at higher frequencies in urban areas (high noise-polluted areas), which has been interpreted as an adjustment to reduce acoustic masking by low-frequency man-made noise. Furthermore, they may also sing louder (i.e. higher amplitude) when exposed to noise and frequency shifting is thought to be less efficient than merely singing louder. Increasing vocalization amplitude in response to background noise is known as the Lombard effect. It has been suggested that high frequency singing might be a physiological consequence of louder singing, rather than just a a functional adjustment to noise. Cardoso & Atwell 2011 tested whether louder oscine songs are sung at higher frequency (central tenet of the hypothesis) using the dark-eyed junco, Junco hyemalis thurberi. The frequency bandwidth of songs and syllables increased with amplitude, involving lower minimum frequency in louder songs and syllables. Therefore, louder singing does not explain the higher minimum frequency of urban dark-eyed juncos. Amplitude and peak frequency were weakly positively related across but not within songs, suggesting that increased frequency is not an obligatory outcome of singing louder. Instead, birds may adjust both amplitude and frequency in response to changing noise or motivation across songs. Their results suggest that adjustments in song frequency and amplitude are largely independent and thus can be complementary rather than alternative vocal adjustments to noise. They discuss oscine vocal physiology and details of the behaviour of urban birds, both of which we argue are consistent with the increased frequency of urban birdsong generally being a functional adjustment to noise, rather than a consequence of singing louder.