Abstract
IntroductionUrbanization can considerably impact animal ecology, evolution, and behavior. Among the new conditions that animals experience in cities is anthropogenic noise, which can limit the sound space available for animals to communicate using acoustic signals. Some urban bird species increase their song frequencies so that they can be heard above low-frequency background city noise. However, the ability to make such song modifications may be constrained by several morphological factors, including bill gape, size, and shape, thereby limiting the degree to which certain species can vocally adapt to urban settings. We examined the relationship between song characteristics and bill morphology in a species (the house finch, Haemorhous mexicanus) where both vocal performance and bill size are known to differ between city and rural animals.ResultsWe found that bills were longer and narrower in more disturbed, urban areas. We observed an increase in minimum song frequency of urban birds, and we also found that the upper frequency limit of songs decreased in direct relation to bill morphology.ConclusionsThese findings are consistent with the hypothesis that birds with longer beaks and therefore longer vocal tracts sing songs with lower maximum frequencies because longer tubes have lower-frequency resonances. Thus, for the first time, we reveal dual constraints (one biotic, one abiotic) on the song frequency range of urban animals. Urban foraging pressures may additionally interact with the acoustic environment to shape bill traits and vocal performance.Electronic supplementary materialThe online version of this article (doi:10.1186/s12983-014-0083-8) contains supplementary material, which is available to authorized users.
Highlights
Urbanization can considerably impact animal ecology, evolution, and behavior
We observed an increase in minimum song frequency of urban birds, and we found that the upper frequency limit of songs decreased in direct relation to bill morphology
These findings are consistent with the hypothesis that birds with longer beaks and longer vocal tracts sing songs with lower maximum frequencies because longer tubes have lower-frequency resonances
Summary
Urbanization can considerably impact animal ecology, evolution, and behavior. Among the new conditions that animals experience in cities is anthropogenic noise, which can limit the sound space available for animals to communicate using acoustic signals. Many species suffer from anthropogenic disturbance, leading to depleted urban biodiversity, some populations thrive and expand in cities [3,6] Factors such as human activity [7], pollution exposure [8], artificial lighting [9], elevated temperature [10], and food and water availability [11] can directly impact wildlife success in urban areas [12]. Among the unique conditions that animals experience in urban habitats, ambient city noise is key because it can limit the sound space available for animals to Recent studies have shown that some bird species adjust their song characteristics − by increasing their minimum frequency − to be heard in a noisy urban environment [13,18,19,20]. The vocal tract (i.e. trachea, larynx, and bill) acts as a resonance chamber for the sound frequencies produced by the syrinx [26,27,28], and subtle variation in bill size/shape and vocal tract morphology affects sound production [22]
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