In penguin species, mates or parents and chicks recognise each other in the hubbub of the colony using mainly acoustic cues, via the display call. From their ability to recognise a particular call in the continuous background noise, it is assumed that they use peculiar strategies of coding/decoding. We have studied these strategies by playing-back modified display calls to two species of penguins. One, the adelie penguin (AP), has a nest which serves as a meeting point, and the other, the king penguin (KP), has no nest site and consequently it has no possibility to use visual landmarks. Both species exhibit signals that are highly redundant in the temporal and frequency domains and have sharp amplitude changes which confer to the call a maximal locatability. As in most other birds, the AP can discriminate a signal at the same intensity as the noise, but the KP is able to identify an individual call when its level is well below the level of the noise generated by other calls (cocktail party effect). According to the theory, two processes facilitate the detection of a signal embedded in the noise: the frequency-band analysis and the temporal analysis of amplitude or frequency modulations. Our experiments demonstrate that the vocal signature of the AP corresponds to the first process and that of the KP to the second. The differences in the strategies of coding-decoding are discussed with respect to the territorial habits and the environmental constraints of both species.
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