Complex modulation of singing behavior by testosterone in an open-ended learner, the European Starling

In many temperate zone songbird species males only produce song during the breeding season, when plasma testosterone (T) levels are high. Males of some species sing throughout the year, even when T levels are low, indicating a dissociation between high T levels and song rate. Given that few studies have taken advantage of these species, we compare here song traits expressed under high versus low T concentrations and we study the role of testosterone in adult song learning in the European Starling, an open-ended learner in which repertoire size dramatically increases with age. We performed a detailed comparison of song complexity and song rate between fall and spring in 6-year-old intact male European starlings. In parallel, we investigated whether potential seasonal changes were regulated by the gonadally induced increase in plasma T, by comparing seasonal changes in intact and castrated males of the same age (castrated as juveniles during their first fall) and by subsequently experimentally elevating T in half of the castrated males. While song rate and stereotypy did not differ between intacts and castrates or between fall and spring, both groups increased their average song bout length from fall to spring, but only intact males increased their repertoire size, indicating that effects of seasonal T changes differ between song traits. Intact males overall displayed a larger song repertoire and a longer bout length than the castrates, and implantation with T caused a turnover in repertoire composition in castrates. However, as the castrates had never experienced high T levels and yet displayed a markedly higher repertoire size than that of typical yearling males, this suggests that the progressive increase of song repertoire with age in male starlings is not dependent on gonadal T, although it may be T-enhanced.