Which birds adjust the frequency of vocalizations in urban noise?

Urban environments present an opportunity to study the evolution of animal communication in acoustically novel habitats. Several species of birds raise the minimum frequency of vocalizations in urban noise, which is louder at lower frequencies. We recorded 12 species of birds in urban and nonurban environments, and tested whether the extent to which different species raise their minimum frequency is related to the frequency range they use. We found that raising the minimum frequency is common in urban birds and is not restricted to passerine song, but also occurs in other vocalizations of passerines and parrots. There was a strong curvilinear relation between the extent to which urban birds raise the minimum frequency and the typical minimum frequency of the species: species with intermediate minimum frequencies (around 1-1.5 kHz) raised the frequency more than species with either higher or lower minimum frequency. This suggests that high-frequency species, which are less affected by urban noise, do not need to adjust the frequency of vocalizations as much. It also suggests that for species with very low frequencies, in a frequency range where urban noise augments exponentially, increasing frequency may not overcome masking by noise efficiently, and these species may use different adaptations. This indicates that frequency differences between species influence the way in which they respond to the same communication problem (masking by low-frequency noise), and possibly also the subsequent evolution of acoustic signals. (C) 2009 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.