In birds, testosterone (T) regulates aggression during the breeding season. However, in some avian species, including European starlings (Sturnus vulgaris), males also display high levels of aggression during the nonbreeding season (autumn and winter), when plasma T levels are low or non-detectable and even after castration. This nonbreeding aggression is generally believed to be independent of testicular T. Moreover, in European starlings several laboratory experiments even indicated that fighting ability was increased rather than decreased by castration. Here, we examined the effect of castration on nonbreeding aggression in captive male European starlings by pairing castrated males with sham-castrated males of the same age in dyadic trials and determining which male was significantly more aggressive when both birds were competing over access for a nestbox to roost in. Birds were (sham) castrated on 9 October and the dominance trials were run between December 2 and February 7. In adults, we found no evidence that castration affected the frequency of nonbreeding aggression, despite the fact that T-levels in castrated males were significantly lower than in sham-castrated birds. In contrast, in yearling males that were castrated at the age of 4-5 months, castration significantly decreased nonbreeding aggression, although T-levels were only marginally decreased by the surgery. The maintenance of aggressive behaviour after castration in adults may be the result of experience or of the action of nongonadal sex steroids. The marked decrease in aggressive behaviour observed in castrated yearlings might be due to the fact that castration at this young age could have inhibited the ontogenetic development of behavioural and physiological mechanisms requiring signals from the gonads. Taken together, these results indicate that the effects of castration on nonbreeding aggression in male European starlings markedly depend on the age at the time of castration.