To examine the ethological role of signal composition in the distress calls of starlings (Sturnus vulgaris), we conducted a series of playback experiments in the field. Earlier researches revealed that frequency modulations are important features of distress calls for triggering the responses of starlings. Another feature, the harmonic structure, was thought to play a role in the coding/decoding process of the call. The aim of this work was to investigate the role of this structure. Four points emerge from these experiments: 1) Ethological message is effectively encoded in the harmonic structure. 2) Wide-band frequency spectra are preferred to narrow-band spectra. 3) Starlings examine the width of the spectrum but not the interval between frequency bands. 4) Spectra with a high intensity upper part produce a stronger response than other types of spectra. Such a coding/decoding process is discussed regarding transmission of information at long range, the ability to appreciate distance, to locate the caller and the possibility of grading the information.