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The Role of Frequency Modulation in the Process of Distress Calls Recognition By the Starling (Sturnus Vulgaris)

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By way to examine the hypothesis that some parameters of the frequency modulation (FM) are the key features of the process of decoding of birds' distress calls, the reaction of starlings (Sturnus vulgaris) to various synthetic, frequency modulated, calls were tested. Playback sessions were performed in the field. Three series of experiments were carried out. 1) In the first series, we made two synthetic signals, each having a very simplified FM structure by comparison with a natural distress call. A simple slope was applied to a carrier frequency which followed the acoustic features of a natural call. The extreme values of the slope were kept within the natural range of the species and they differed only by the sign of the slope. The signal with the positive slope induced positive responses: the one with the negative slope did not. 2) In the second series, we constructed three signals which differed from each other by the rise in frequency, i. e. the gradient of the slope. For two of the signals, this gradient was well outside the natural range for the species. No consistent or significant differences were found between the responses to these different signals. 3) In the third series, we synthesized six signals with sections of increasing, decreasing and constant frequency, in different combination. Signals which combined increasing and constant frequency sections elicited positive responses, independently of the order of the sections. In contrast, signals with decreasing and constant frequency sections were not recognized. When increasing and decreasing sections were present together in the same signal, the birds reactions depended on the order of the successive sections. If the increasing section preceded the decreasing one, starlings exhibited in most cases positive responses. In the contrary case, they responded weakly to the signal. Three points emerge from these series of experiments: 1) the starling follows a very simple law for the decoding of its distress vocalizations: an ascending FM (positive slope) is required to evoke positive responses; 2) the decoding law is very strict with regard to the sign of the slope: falling frequencies lead to a rejection of the signal by the starling. This rejection is particularly obvious when the entire signal presents a descending FM or when the signal begins with a decreasing FM; 3) the decoding law is very flexible with regard to the value of the rise in frequency and to the form of the ascending FM. These values can fall well outside the range found in natural distress calls, yet still be recognized. Thus, a phenomenon of asymmetry between the coding and decoding possibilities appears in the starling distress calls.

Affiliations: 1: Laboratoire d'Ethologie Expérimentale de St. Lucien, C.N.R.S. UA 667, 28210 Nogent-Le-Roi, France

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