The study of vocal amplitude thus yields new insights not only into the vocal mechanisms of song production but also on the constraints imposed on singing and the role of signal intensity during male-male competition. Further research on the causes and consequences of singing loudly will shed more light on processes of communication in territorial birds. The mechanisms of amplitude variation revealed so far reflect only some aspects of the whole phenomenon. For instance, an increase in vocal amplitude during interactions seems not to be always adaptive. Under certain circumstances, such as specific aggressive contexts or during courtship, birds decrease song intensity (Dabelsteen et al. 1998). Thus the issue of vocal amplitude adjustment in birds is complex and we need more studies examining the social influences on song level variation and the impact of sexual selection, before we can fully understand the role of vocal amplitude in birdsong.
ACKNOWLEDGMENTS
I thank Dietmar Todt and Henrike Hultsch for their support. In addition, I appreciate the assistance in the field by Tina Sommer and Roger Mundry and the help with the pattern analysis by Nicole Geberzahn and Friederike Kiesselbach. I am also grateful to Peter Marler and Nicole Geberzahn for their comments on the manuscript. Finally, I thank Jacques Vielliard and Maria Luisa da Silva for the kind invitation to the XIX IBAC.
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