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Monogamous male birds typically allocate less e¡ort to courtship and more to parental behaviour than males of polygynous species. The seasonal pattern of testosterone (T) secretion varies accordingly. Monogamous males exhibit a spring peak in plasma T followed by lower levels during the parental phase, while males of polygynous species continue to court females and maintain T at higher levels. To determine whether testosterone underlies the trade-o¡ between mating and parental e¡ort, we treated male darkeyed juncos (Junco hyemalis) with exogenous T and compared the reproductive success (RS) of T-treated males (T-males) to that of controls. T-males had lower apparent annual RS than controls, probably because elevated T reduced parental care. Nevertheless, annual genetic RS of the treatment groups was similar because (i) T-males su¡ered fewer losses in genetic RS due to extra-pair fertilizations (EPFs), and (ii) T-males gained more genetic RS through their own EPFs. This is the ¢rst hormonal manipulation of an avian phenotype shown to in£uence male RS through EPFs. Together with other studies, it suggests that testosterone may have mediated the evolution of inter-and intraspeci¢c di¡erences in allocation of reproductive e¡ort to mate attraction and parental care.

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