Testosterone and territorial behaviour in sedentary and migratory sparrows

Free-living male song sparrows, Melospiza melodia morphna, showed marked aggressive behaviour in response to simulated territorial intrusions throughout most of the year. Only during the prebasic moult, and during inclement winter weather, was territorial aggression suppressed. In contrast, free-living male white-crowned sparrows, Zonotrichia leucophrys pugetensis , showed the highest behavioural responses to simulated territorial intrusions in spring when territories were established. Responses then declined as the breeding season progressed, reaching a minimum during the prebasic moult. The latter species migrates to wintering grounds and does not show territorial aggression until the following spring. Plasma levels of testosterone paralleled changes in territorial aggression in white-crowned sparrows, but not song sparrows. The resurgence of autumnal territoriality was independent of any increase in circulating testosterone levels. Males of both species showed increases in testosterone levels in response to simulated territorial intrusions, but this was only significant in white-crowned sparrows. However, male song sparrows had higher levels of testosterone in spring in areas with a high density of territories than males in areas with a low density of territories. These data suggest that although territorial behaviour in the non-breeding season was independent of testosterone in song sparrows, such behaviour in the breeding season may be influenced by this steroid hormone. Finally, it appears that young, first summer, song sparrows were also able to establish a territory in early autumn with no changes in testosterone or luteinizing hormone.