Many aspects of the social behaviour of birds are mediated by vocal displays, and variation in song output or song structure conveys differentinformation to receivers. After nest construction begins, when vermilionflycatcher (Pyrocephalus rubinus) females are potentially fertile, malesincrease their song rate during the dawn chorus. A previous study failedto give evidence that males discriminate among song rates. However,males sing in sequences of songs (song bouts), and an increase in songrate may be achieved by increasing the number of bouts, the number ofsongs in each bout (bout size) or both. Studying a vermilion flycatcherpopulation in Mexico City, we evaluated whether dawn song rate isrelated to song bout size or to number of bouts. Bout size correlatedwith song rate and differed among males. We hypothesized that longerbouts are more threatening signals than shorter ones and predicted astronger response by males towards the former. We exposed each maleto three playback treatments: (1) Long song bout (Long), in which wereplied to the male with twice the number of songs he sang in the bout,(2) Short song bout (Short), in which we played half the number ofsongs sung by the male and (3) Control, this was the same as the Longtreatment but we used songs from a related species, the tropical kingbird(Tyrannus melancholicus). Males responded with a higher proportion ofcalls near the speaker when exposed to the Long treatment than duringthe Short or Control treatments, indicating that they discriminate amongsong bouts differing in size, that they may perceive longer bouts as morethreatening and that they use calls rather than songs to address threatening situations. Our results suggest that song bout size is a relevantsong attribute that conveys information during intrasexual interactions.
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