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Computer-aided acoustic analysis of complex bird calls [abstract]

J. Böhner & K. Hammerschmidt (1996). Computer-aided acoustic analysis of complex bird calls [abstract]. Bioacoustics, Volume 6 (4): 313 -314



An important step in bioacoustic research was the development of the sound spectrograph, enabling a detailed description of the frequency-time structure of acoustic events. However, an unambiguous determination especially of frequency parameters through direct measurements from the sonagram is often difficult and usually possible only in pure-tone sounds or when a few clear harmonics are present. Especially in non-oscine bird species, calls often contain "noisy'', i.e. non-harmonic, frequency components or a large number of harmonic frequency bands, of which the relative amplitudes are not clearly recognizable in sonagrams. To deal with these problems, a computer program (LMA 3.0, Hammerschmidt 1990) was developed which determines up to 85 frequency, amplitude and time parameters on the basis of frequency-time spectra. For example, LMA calculates the number and positions of dominant frequency bands above a certain amplitude threshhold for all time segments of a call. The exact value of this threshhold is variable and depends on the amplitude distribution in the respective call segment. Further parameters characterizing the amplitude distribution are calculated, like the median and the 1st and 3rd quartile of the overall amplitude. Such parameters are especially relevant if clear amplitude maxima like dominant frequency bands are missing. To describe temporal changes. characteristic values of the calculated parameters in the course of the call are then determined (e.g. start value, minimum, maximum, modulation). As examples we describe two studies on vocalizations of the hooded crow Corvus corone cornix and the bald ibis Geronticus eremita. In both species, certain call types obviously important in intraspecific communication have a complex structure including tonal and atonal parts. We demonstrate that especially parameters characterizing the amplitude distribution are individually distinctive and, in the case of the bald ibis, can also indicate the respective behavioural context of calling.