We examined two measures of immune function (bactericidal capacity of the blood plasma and a pro-inflammatory response to an injection of phytohemagglutinin) and baseline levels of corticosterone in wild adult male Brown-headed Cowbirds (Molothrus ater) to determine whether the size of the song repertoire, a fixed or “static” signal, can be linked to current condition. We hypothesized that static signals and immune function are positively correlated with males' quality because high-quality males should exhibit relatively large sexual signals and also maintain high levels of immune function. In contrast to our primary prediction, we found that song-repertoire size and bactericidal capacity were negatively correlated. We also found that repertoire size and corticosterone levels were positively correlated and that there was no relationship between corticosterone level and either measure of immunity. The positive correlation between corticosterone level and repertoire size suggests that males with larger repertoires expend more energy than do males with smaller repertoires and that the negative correlation between repertoire size and bactericidal capacity is mediated via activity levels, because corticosterone does not appear to suppress immune function directly. If repertoire size indicates a male's quality, these results imply that the level of immune function of high-quality males is not high at all times and that increased energy expenditure related to breeding may draw resources away from immune function.
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Vol. 115 • No. 2