Effects of an immune challenge on multiple components of song display in barn swallows Hirundo rustica: Implications for sexual selection

Bird song is believed to honestly reveal male quality including the ability of singers to face parasitism. In a natural population of barn swallows Hirundo rustica, we experimentally imposed a cost on song production by an immune challenge. We therefore vaccinated a group of reproducing males with an antigen (Newcastle disease virus), and injected phosphate-buffered saline to a control group. Immune challenge significantly reduced one song feature, rattle duration. This decrease was related to male quality, as measured by tail length, because males with short tails reduced the duration of their rattle significantly more than males with long tails. In addition, another song feature, strophe duration, decreased in the control group, while it remained constant in the challenged group. Duration of the rattle has previously been found to be positively related to testosterone level, and it may hence reflect male competitive ability. Thus, male barn swallows may not have the potential to produce long rattles when their immune system is challenged. By maintaining their strophe duration after an immune challenge, males may compensate for the decrease in rattle duration. Our results suggest that different song features may convey different types of information, and that barn swallow song, in particular their rattle, may reliably reveal information about activation of the immune system.