Factors intrinsic or extrinsic to individuals, such as their quality or the quality of competition in their social environment, can influence their communication signalling effort. We hypothesized that telencephalic monoamine secretion mediates the effects of a male’s own quality and quality of his social environment on his sexual signalling effort. The duration of a male European starling’s (Sturnus vulgaris) principal sexual signal, his song, positively correlates with several aspects of his quality, including his reproductive success, immunocompetence, and ability to attract mates. Therefore, the length of songs to which he is exposed reflects, in part, the quality of competition in his social environment. We manipulated the quality of the competitive environment by exposing male starlings to long or short songs for one week. We measured the length of songs produced by experimental males to gauge their quality, counted the number of songs they produced to gauge singing effort, and quantified telencephalic monoamine metabolism using HPLC. Singing effort increased with the length of the males’ own songs and with the length of songs to which we exposed them. Norepinephrine metabolism in Area X of the song control system was negatively correlated with the subjects’ mean song length and singing effort. Serotonin metabolism in the caudomedial mesopallium of the auditory telencephalon increased with the length of songs to which we exposed the subjects and with their singing effort. This raises the hypothesis that serotonin and norepinephrine secretion in the telencephalon help mediate the effects of extrinsic and intrinsic factors on signalling effort
