The Low-frequency Acoustic Structure of Mobbing Calls Differs Across Habitat Types in Three Passerine Families

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Animal Behaviour



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The acoustic adaptation hypothesis predicts that animals should adaptively respond to the transmission properties of the habitat in which they communicate. Although there have been many tests of the acoustic adaptation hypothesis with birdsong, there have been very few tests with different types of bird vocalizations. Here I tested the predictions of the acoustic adaptation hypothesis with avian mobbing calls produced in closed, open and urban habitats in three families of passerine birds. I also controlled for body size and phylogeny since these are known to influence acoustic characteristics of vocalizations. I found that body size was important in duration and frequency measurements of mobbing call acoustic structure. Phylogeny explained acoustic variation in only some acoustic variables measured. I also found only the two low-frequency measurements to differ across habitats. First, 5% frequency (a measurement of low-frequency energy distribution) differed between species classified as occurring in predominately open or closed habitats, with species classified as closed having lower 5% frequency than species classified as open. This finding supports the prediction that species in closed habitats will have lower low frequencies than species in open habitats. Additionally, I found that species classified as urban had a lower minimum frequency. This is in direct opposition to the prediction of the acoustic adaptation hypothesis and previous findings for birdsong, where species appear to shift lower minimum frequencies upward, likely to avoid masking by anthropogenic noise. To confirm this finding, I also measured low frequency using power spectra with an amplitude threshold (i.e. threshold method) and confirmed the same result: species classified as urban had lower minimum frequencies than species classified as open or closed.


Acoustic adaptation hypothesis; By-eye method; Low frequency; Mobbing calls; Phylogenetic comparative analysis; Threshold method


Poultry or Avian Science



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