Male song sparrows (Melospiza melodia morphna) of western Washington State show year-round territoriality. Although territorial aggression during the breeding season was accompanied by high circulating levels of luteinizing hormone (LH) and testosterone (T), similar aggression in the non-breeding season occurred when gonadal hormone levels were basal. Experimental removal of territorial males in autumn resulted in new males taking over the territory within a few days. These males had basal levels of LH and T despite establishing a new territory and the accompanying high level of aggression. There was also no difference in plasma levels of corticosterone in replacement versus established territorial males suggesting that social stress was not a factor. To test the possibility that low levels of gonadal steroids, or potentially some gonadal hormone not measured here, might regulate territorial aggression in the non-breeding season, free-living male song sparrows were castrated in early autumn and their aggressive territorial behavior was quantified by a simulated territorial intrusion (STI). Castrates showed the same aggressive responses to STI as controls indicating that establishment and maintenance of a territory was not dependent upon hormones of gonadal origin. Castrated males retained territories throughout the autumn and winter and even into the following breeding season. Further experiments were conducted to establish a role, if any, for T in territorial behavior in this species. During the breeding season when males had high levels of T, the responses to STI continued after the stimuli (caged conspecific male and tape-recorded songs) were withdrawn. Some males continued to patrol the territory and sang for long periods after STI. In contrast, males exposed to STI in autumn showed a strong aggressive response during the challenge, but then territorial aggression abated rapidly when the stimuli were withdrawn. Implants of T into males in autumn reinstated the high level of aggression after withdrawal of STI in a manner similar to that seen during the breeding season. These data suggest that in M. m. morphna T does not activate territorial aggression per se, but may increase the intensity of an aggressive response to STI, and persistence of the behavior after STI is withdrawn.
