Amplitude spectra of Corncrake calls: what do they signalise?

In: Animal Biology
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—The territorial call of corncrake Crex crex males is a disyllabic, loud, pulse repetition signal repeated in long series at night. It seems to be relatively simple as it lacks the repertoire variation typical of passerines, but still seems to be the equivalent of a territorial and/or advertisement song. In this paper we tested: (1) whether the pattern of energy distribution within the call (i. e. amplitude spectrum) is individually invariant; (2) whether it could be a signal used in an individual recognition system; and (3) whether it is a sexually selected honest trait related to male body size. We found that amplitude spectra varied more between than within males, but it is rather unlikely that energy distribution is a single feature encoding identity of a male. We found some weak correlations between body size parameters and variables describing amplitude spectra, either supporting or contradicting the expected relationship (i. e. larger male — more energy in lower frequencies). We also found that the frequency of maximal amplitude, even within a single call, may rapidly change by a few kHz, which is most probably an effect of head and body movements, not changes in sound production itself. The conclusion is that all features of corncrake calls, including energy distribution, indicate that this signal has evolved under a strong pressure towards precise localisation of a sender, which is understandable as corncrakes inhabit dense vegetation and call almost exclusively at night.

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