![]() Androgenic control of vocal and non-vocal territoriality is more evident in species that are territorial and sing only during the breeding season. Thereby, testosterone can influence several reproductive traits, including vocal and non-vocal territorial behavior. ![]() In many temperate species, male birds sing and establish a territory at the onset of the breeding season when circulating levels of testosterone are high (e.g. Our study contributes to the understanding of the complex comparative neuroendocrinology of song birds in the wild. This suggests that seasonal variations in circulating testosterone concentrations and brain sensitivity to androgens is widely uncoupled from territorial behavior and song activity but might still affect song pattern. Our results demonstrate seasonal modulation of song, circulating testosterone levels, and brain sensitivity to androgens, but a year-round persistency of territorial behavior and estrogen receptor expression in all life-cycle stages. Territorial behavior and song rate correlated with the expression of steroid receptors in hypothalamic areas, and in the song control nucleus lMAN. Nevertheless, some song spectral features and the song repertoire differed between breeding and nonbreeding. The expression of androgen receptors in the song control nucleus HVC was lower during molt when birds do not sing or sing rarely, but similar between the breeding and the nonbreeding stage. But plasma testosterone was higher during breeding than during the other stages, similar to androgen receptor and aromatase expression in the preoptic area. Territorial behavior and estrogen receptor expression in hypothalamic areas did not differ between stages. Concurrently, we measured changes in circulating testosterone levels, as well as the mRNA expression of androgen and estrogen receptors and aromatase in the preoptic, hypothalamic and song control brain areas that are associated with social and vocal behaviors. To assess the effects of seasonal androgenic action on territorial behavior, we analyzed vocal and non-vocal territorial behavior in response to simulated territorial intrusions (STIs) during three life-cycle stages in free-living male black redstarts: breeding, molt and nonbreeding. The connection between testosterone and territoriality in free-living songbirds has been well studied in a reproductive context, but less so outside the breeding season.
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