The use of bioacoustic parameters for evaluating bird relationships opened up exciting perspectives in the 1960s, when the appearance of high-fidelity portable recorders allowed for' the gathering of wildlife sound signals on a large scale. However, the lack of a good understanding of the processes of animal sound communication frustrated these initial expectations. Another limiting factor resides in the lack of a sufficient data-base of well-documented recordings: in spite of the tremendous growth of many sound archives around the world, few people realize the still very great need to collect, deposit, document, edit and exchange many more recordings. On the other hand, it recently became clear that the situation is ripe for new efforts in this field today, when molecular phylogeny upsets the classical anatomo-morphological one; bioacoustical phylogeny, quantifying biologically functional parameters, might help solve many pending cases, if adequately used. That was the purpose of the discussion I launched during the XXI International Ornithological Congress in Vienna last year. What has been established is the starting point for further discussions and actions. As it appears from my report (Bioacoustics 6(2), 171-174, 1995), unanimous resolutions were reached along two basic lines. First, there is an absolute need for larger, better documented and effectively inter-connected data-bases, i.e. sound archives, if one wants to comply with the following premises. These premises, our second resolution agreed upon, are:
- because of the "species-specific recognition'' (SSR) biological value of the functional song in birds, phylogenetic significance will be around the species level, although common patterns might be evidenced at generic or higher levels,
- the specific "acoustic communication system'' (ACS) must be understood (i.e. repertoires as complete as possible and the biological function of each signal are to be documented), in order to differentiate between analogous vs. homologous sound structures (phylogenetic comparisons are valid only between homologous parameters),
- ontogeny of the acoustic signals must be identified as directly as possible, not only by indirect clues (for instance, lack of regional variation in genetically determined songs).
Now that these prerequisites are recognized, the question is how to quantify and evaluate these bioacoustic parameters. Digital signal processing (DSP) is a powerful tool for quantifying sounds, but care is needed to identify the useful parameters and evaluate them accordingly. Nevertheless, what seems to me to be the great challenge is to identify the polarisation of the state of bioacoustic parameters, i.e. if plesio- or apomorphic, and so which, in a species-group, are the ancestral species and which the derived ones. I will try to illustrate some cases, using my studies on some Brazilian birds in various families. Is, for instance, an harmonic structure more evolved than the pure whistle of a homologous sound by a closely related species? Or, is continuous staccato more primitive than a temporal pattern in a sequence of the same note? So, this Symposium should open the next round of discussion on bioacoustical phylogeny: how to recognize synapomorphies?