Any acoustic signal used in animal communication can be described by three physical dimensions: frequency, time and amplitude (“loudness”). Their values differ between signals and may be shaped by natural and sexual selection. It is well known that bird song has an important role in mate attraction and a number of studies show how the time pattern and frequency structure of male advertisement songs influence female mating behaviour. However, the role of song amplitude in female choice in birds has been neglected. In several species, including the zebra finch (Taenopygia guttata), considerable differences in song amplitude between males have been found, which makes this signal trait a potential candidate for sexual selection. We tested the song preferences of female zebra finches in an operant task with song playback as the sole reward. The subjects were kept singly in soundproof cages and could trigger song playback by pecking at illuminated keys. Male songs at three different amplitudes within the natural range were provided as well as a degraded song at intermediate amplitude which had been recorded from several meters distance. Our results suggest that females prefer loud songs over low amplitude songs and that degradation does not influence the attractiveness of a song. Loud songs may be more costly to produce than soft songs and loudness may thus signal male quality. On a proximate level, high intensity songs may be more efficient in stimulating the receiver’s sensory apparatus, causing auditory neurons to fire at higher rates, and therefore may be preferred by females. Degradation seems not to affect the song features that signal species identity and male quality or detrimental and positive effects of degradation on zebra finch songs may be balanced.
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