A test of the mechanistic process behind the convergent agonistic character displacement hypothesis

In this era of rapid global change, understanding the mechanisms that enable or prevent species from co-occurring has assumed new urgency. The convergent agonistic character displacement (CACD) hypothesis posits that signal similarity enables the co-occurrence of ecological competitors by promoting aggressive interactions that reduce interspecific territory overlap and hence, exploitative competition. In northwestern Switzerland, ca. 10% of <i>Phylloscopus sibilatrix</i> produce songs containing syllables that are typical of their co-occurring sister species, <i>Phylloscopus bonelli</i> ("mixed singers"). To examine whether the consequences of <i>P. sibilatrix</i> mixed singing are consistent with CACD, we combined a playback experiment and an analysis of interspecific territory overlap. Although <i>P. bonelli</i> reacted more aggressively to playback of mixed <i>P. sibilatrix</i> song than to playback of typical <i>P. sibilatrix</i> song, interspecific territory overlap was not reduced for mixed singers. Thus, the CACD hypothesis was not supported, which stresses the importance of distinguishing between interspecific aggressive interactions and their presumed spatial consequences.