Short-term changes in plasma levels of hormones during establishment and defense of a breeding territory in male song sparrows, Melospiza melodia.

When territorial male song sparrows are captured and removed from their territories, previously unmated and nonterritorial males will take over those vacant territories within 12-72 hr. Plasma levels of testosterone are elevated in these replacement males as well as in their neighbors. Since the latter already have territories, it is suggested that the agonistic interactions over territory boundaries, or behavioral stimuli from challenging males, rather than ownership of a territory per se, stimulates secretion of testosterone. To test this hypothesis further, male song sparrows were challenged by experimental simulation of a territorial intrusion. This procedure involved placing a caged male song sparrow in the center of the subject's territory and playing tape recorded conspecific song through an adjacent speaker. Responding males were then captured at intervals after onset of the intrusion. Plasma levels of testosterone were significantly higher in males exposed to experimental territorial intrusion than they were in controls, supporting the hypothesis that behavioral stimuli emanating from an intruding male can act as supplementary information stimulating secretion of testosterone.