Control of territorial aggression in a changing environment.

A sedentary population of song sparrows, Melospiza melodia morphna, in western Washington State shows year-round territoriality but in different contexts. During the breeding season (March-August), all defend multiple purpose territories as monogamous pairs, or alone, but only about 30% of individuals remain on the same territory during the nonbreeding season (September-February). During the breeding season, territorial behavior is tightly correlated with circulating levels of luteinizing hormone (LH) and testosterone, but identical behaviors during the nonbreeding season appear to be expressed independently of gonadal hormones. First summer males establish territories with basal plasma levels of LH and testosterone. Castrated male song sparrows defend territories equally well as intact males in autumn and continue to do so even through onset of the breeding season. It can therefore be questioned whether testosterone has any role in activation of territorial behavior in this species. Experiments in which testosterone levels were manipulated indicate that testosterone does not simply activate territorial behavior but enhances "persistence" of aggression during the breeding season. It is suggested that although territorial behavior may be expressed year-round, the context, neural, and hormonal bases of territoriality may change dramatically.