title: Song characteristics track bill morphology along a gradient of urbanization in house finches (Haemorhous mexicanus).
abstract: BACKGROUND
  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. 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.
  
  
  RESULTS
  We 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.
  
  
  CONCLUSIONS
  These 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.
authors.affiliation: School of Life Sciences, Arizona State University, Tempe, AZ 85287-4501 USA ; Present address: School of Biological Sciences A08, University of Sydney, Sydney, NSW 2006 Australia.
authors.forename: Mathieu
authors.initials: M
authors.lastname: Giraudeau
authors.affiliation: Department of Biology, The Citadel, Charleston, SC 29409 USA.
authors.forename: Paul M
authors.initials: PM
authors.lastname: Nolan
authors.affiliation: Department of Biology, The College of Charleston, Charleston, SC 29424 USA.
authors.forename: Caitlin E
authors.initials: CE
authors.lastname: Black
authors.affiliation: Global Institute of Sustainability & School of Sustainability, Arizona State University, Tempe, AZ 85287-5402 USA.
authors.forename: Stevan R
authors.initials: SR
authors.lastname: Earl
authors.affiliation: Graduate School of Life and Environmental Sciences, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba-shi, Ibaraki 305-8572 Japan.
authors.forename: Masaru
authors.initials: M
authors.lastname: Hasegawa
authors.affiliation: School of Life Sciences, Arizona State University, Tempe, AZ 85287-4501 USA.
authors.forename: Kevin J
authors.initials: KJ
authors.lastname: McGraw
keywords: Bill shape; Noise pollution; Singing behavior; Urban impacts; Vocal communication