In penguins, individual recognition is observed between mates and between parents and chicks. Over five years we tested their peculiar strategies of coding-decoding by playing-back modified display calls to 6 species of penguins in Australia (Little Penguin, Eudyptula minor), in Antarctica (Adelie Penguin, Pygoscelis adeliae, and Emperor Penguin, Aptenodytes forsteri)and in subantarctic islands (King Penguins, Aptenodytes patagonicus, Macaroni Penguin, Eudyptes chrysolophus, Gentoo Penguin, Pygoscelis papua). All use only vocal cues but, in territorial species, the nest serves as a meeting point. In large species as the king and the emperor penguin which does not have a nest, the brooder carries the egg or the small chick on the feet, while the large chick has to be located in the noisy colony without this topographical cue. According to theory, to extract a signal from the back- ground noise, animals analyse either frequency bands of the call or frequency/ time modulations (= temporal analysis of amplitude). The first one, used by non-nesting penguins, is easy to produce but costly in terms of analysis time. The second one, used by nesting penguins, is a vocal signature that is fast to analyse but costly to produce. For a non-nesting species, it is particularly advantageous to locate immediately the partner in a noisy crowd on the move. To conclude, the frequency analysis solves the relatively easy problem of individual recognition in nesting birds, while the complex code of the two non- nesting species is an adaptation to extreme acoustic .and breeding conditions.