Acoustic convergence between two nocturnal burrowing seabirds:: experiments with a penguin Eudyptula minor and a shearwater Puffinus tenuirostris

The evolution of acoustic signals is influenced by environmental constraints. We studied two sympatric but unrelated seabirds: the Little Penguin Eudyptula minor and Short-tailed Shearwater Puffinus tenuirostris, to examine the degree to which similarities in their ecology had led to convergence in their calls. Both species nest in burrows in Southern Australia and, at night, are highly vocal and territorial. First, we analysed the physical characteristics of the territorial call. Secondly, we studied the transmission of calls through burrows and varying distances through vegetation. Thirdly, we used playback experiments of natural signals to demonstrate that the response disappears between 4-8 m, and of modified signals to understand the coding-decoding process linked to the territorial function of the call. The structure of the territorial calls of the two species clearly differs, but both species produce a succession of gaps in amplitude and frequency, and a high degree of redundancy. Our experiments show that, to decode the territorial message, birds pay attention only to parameters that are less degraded during propagation and ignore fine details of structure that are quickly degraded, even at relatively short distances (< 8 m). In both species, territorial information is mainly conveyed by the rhythmic succession of two sounds (syllables or subsyllables), birds paying attention to the FM structure of these successive sounds but not to the AM. This convergent coding is adaptive in that it reduces the possibility that the meaning may be distorted by interference from noise and acoustic screening.