A paper by Jakub Szymkowiak and Lechosław Kuczyński (Population Ecology Lab) about signaling function of male song rate during territorial contests in the wood warbler (Phylloscopus sibilatrix) has been published in the current issue of Journal of Avian Biology. Using playback experiments, Authors showed that wood warbler males respond differentially to simulated territorial intrusions depending on the song rate of an intruder, increase singing rates when challenged, and that this behavior predicts the strength of male aggressive response towards intruders. These results suggest that song rate is an aggressive signal during male-male interactions in wood warblers. Moreover, Authors found also high intra-individual repeatability in male aggressive behavior, likely reflecting differences in personality (aggressiveness) or quality of male wood warblers.
You can read the paper here: http://doi.org/10.1111/jav.00969