Conventional communication, like human speech or the bee dance, consists of arbitrary signals associated with meaning through convention. Acoustic conventions seem to be frequent among passerine birds having song repertoire variability, but such behaviors are unknown in nonpasserine species producing simple calls. Because the variability of vocalizations is relatively small in nonpasserine birds, no study has investigated their ability to create conventional signals. We experimentally show here that Corncrake males (Crex crex), modifying rhythm in simple 2-syllable calls, signal their neighbors about their aggressive motivation. Males responded more aggressively (approached the speaker and attacked it) to playbacks with intermittent than with monotonous rhythm, and males calling with a more intermittent rhythm were more aggressive. The presence of convention in this system requires that the production of alternative signals is not linked with differential production costs. In Corncrakes, aggressiveness is signaled by a signal with lower production cost (lower call rate) than the lack of aggressiveness, and our results indicate that the honesty of the rhythm is maintained by a receiver retaliation cost rather than by production costs. Our observations suggest that temporal organization of signals in Corncrakes is an example of syntax, equivalent to a very simple Morse code system.
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