Implanted males (Table 2). However, this effect is not reflected in a higher frequency bandwidth of part B in fall, in contrast to placeboimplanted males in spring (Fig. 4b, Table 2). Furthermore, changes in the frequency bandwidth of part B occur at a far narrower range in fall than in spring (Figs. 1326631 3b and 4b). With regard to the effect sizes (Table 2) we suggest to treat the results on frequency measures in fall with caution.Treatment and Season Affect Song Modulation during Territorial ChallengesAlthough all males (regardless of treatment and season) changed their song in the aggressive context, Flut/Let males in spring and all males challenged during Etermined four weeks later by isolating gastric tissue and using qPCR non-breeding in fall did so to a lesser extent than placebo males during breeding in spring. The changes that we find to be inhibited by the Flut/Let-treatment in spring (i.e. maximum frequency of part A and frequency bandwidth of part B) are similar to the parameters Cucco and Malacarne (1999, ` [47]) found to be characteristic for adult males song as opposed to yearling males’ song. These parallels in acoustic Title Loaded From File features that differ between age-groups [47] and males of different hormonal status (our study) deserve further consideration. Yearlings as well asDiscussionIn this study, we explored the role of testosterone (and its estrogenic metabolites) in modulating song characteristics of black redstarts in a spontaneous and 1379592 a reactive context both during breeding and non-breeding. Territorial males of both treatment groups and in both seasons did change structural song parameters in an aggressive context. In spring, both treatment groupsTestosterone Affects Song Modulationmales with low testosterone levels might fail to produce challenging acoustic features due to lack of experience. Considering that adult male black redstarts (singing `mature song’) in general have a higher reproductive success than yearlings [48,49], we assume that this mature song is indicating a better quality and our Flut/Let-implanted males failed to produce this `mature song’. Thus, context-dependent changes in song structure may indeed reveal information about the quality of the producer. In Flut/Let-implanted males during spring and all males during fall the increase in the number of elements in part A was associated with a decrease in its maximum frequency. Therefore, Flut/Letmales in spring and all males in fall tended to sing this song part with a lower frequency bandwidth during a challenge than during spontaneous song. This might be interpreted as a failure to increase the number of elements and maintain the frequency at the same time in terms of a performance constraint, or alternatively, that Flut/Let-implanted males invested less into the production of these signals than did placebo-implanted birds in spring. Considering that territorial behaviors other than song were not affected by a Flut/Let-treatment in spring (Apfelbeck et al., under revision) it is likely that motivational differences can not exclusively account for our results. In addition, in contrast to placebo-implanted males Flut/Let males did not increase the frequency bandwidth of song part B. Part B consists only of a single noisy song element. Noisy elements are characterized as atonal, non-harmonic sounds occupying a range of frequencies (Fig. 1). There are good reasons to assume that such atonal song elements are not produced by the syrinx but by modulating the airflow in the vocal tract (reviewed in [59]). Accordingly, placebo-implanted.Implanted males (Table 2). However, this effect is not reflected in a higher frequency bandwidth of part B in fall, in contrast to placeboimplanted males in spring (Fig. 4b, Table 2). Furthermore, changes in the frequency bandwidth of part B occur at a far narrower range in fall than in spring (Figs. 1326631 3b and 4b). With regard to the effect sizes (Table 2) we suggest to treat the results on frequency measures in fall with caution.Treatment and Season Affect Song Modulation during Territorial ChallengesAlthough all males (regardless of treatment and season) changed their song in the aggressive context, Flut/Let males in spring and all males challenged during non-breeding in fall did so to a lesser extent than placebo males during breeding in spring. The changes that we find to be inhibited by the Flut/Let-treatment in spring (i.e. maximum frequency of part A and frequency bandwidth of part B) are similar to the parameters Cucco and Malacarne (1999, ` [47]) found to be characteristic for adult males song as opposed to yearling males’ song. These parallels in acoustic features that differ between age-groups [47] and males of different hormonal status (our study) deserve further consideration. Yearlings as well asDiscussionIn this study, we explored the role of testosterone (and its estrogenic metabolites) in modulating song characteristics of black redstarts in a spontaneous and 1379592 a reactive context both during breeding and non-breeding. Territorial males of both treatment groups and in both seasons did change structural song parameters in an aggressive context. In spring, both treatment groupsTestosterone Affects Song Modulationmales with low testosterone levels might fail to produce challenging acoustic features due to lack of experience. Considering that adult male black redstarts (singing `mature song’) in general have a higher reproductive success than yearlings [48,49], we assume that this mature song is indicating a better quality and our Flut/Let-implanted males failed to produce this `mature song’. Thus, context-dependent changes in song structure may indeed reveal information about the quality of the producer. In Flut/Let-implanted males during spring and all males during fall the increase in the number of elements in part A was associated with a decrease in its maximum frequency. Therefore, Flut/Letmales in spring and all males in fall tended to sing this song part with a lower frequency bandwidth during a challenge than during spontaneous song. This might be interpreted as a failure to increase the number of elements and maintain the frequency at the same time in terms of a performance constraint, or alternatively, that Flut/Let-implanted males invested less into the production of these signals than did placebo-implanted birds in spring. Considering that territorial behaviors other than song were not affected by a Flut/Let-treatment in spring (Apfelbeck et al., under revision) it is likely that motivational differences can not exclusively account for our results. In addition, in contrast to placebo-implanted males Flut/Let males did not increase the frequency bandwidth of song part B. Part B consists only of a single noisy song element. Noisy elements are characterized as atonal, non-harmonic sounds occupying a range of frequencies (Fig. 1). There are good reasons to assume that such atonal song elements are not produced by the syrinx but by modulating the airflow in the vocal tract (reviewed in [59]). Accordingly, placebo-implanted.