Distress calls are signals effective over a long distance. They are well known to evoke interspecific reactions. We suggest that the interspecifity phenomenon results from the use of similar laws of decoding by the species concerned. These laws must take into account the transmission channel which always has a great influence on long-range communication. We tested our hypothesis by broadcasting simplified synthetic calls to two species of birds: the herring gull and the starling. The various calls differed in terms of frequency modulation (FM). Two main conclusions emerged from this series of tests:
1) The parameters used for recognition are not sophisticated: a simple slope applied to a carrier frequency that corresponds to the acoustic shape of a distress call is sufficient to infer a distress meaning to the signal. The basic rules are the same for the gull and the starling, with differences only in the acceptance level of the species.
2) The system of recognition is based upon parameters not altered by the environment: the birds make use of the slow frequency modulations (FMI). In contrast, the fast frequency sweeps (FMII) which are modified during propagation do not seem to be utilized. The use of these characteristics of distress calls for recognition allows interspecifity and maximum efficiency for propagation over long distances.