Life-history and hormonal control of aggression in black redstarts: Blocking testosterone does not decrease territorial aggression, but changes the emphasis of vocal behaviours during simulated territorial intrusions

We investigated black redstarts (Phoenicurus ochruros), a species that shows periods of territoriality within and outside of the breeding season. We treated territorial males with an anti-androgen and an aromatase inhibitor during the breeding season to block both the direct and indirect effects of testosterone. Three and ten days after the treatment, implanted males were challenged with a simulated territorial intrusion. The treatment did not reduce the overall territorial response, but it changed the emphasis of territoriality: experimental males invested more in behaviours addressed directly towards the intruder, whereas placebo-treated males put most effort into their vocal response, a component of territoriality that may be primarily directed towards their mating partner rather than the male opponent.In combination with previous findings, these data suggest that overall territoriality may be decoupled from testosterone in male black redstarts. However, high levels of testosterone during breeding may facilitate-context dependent changes in song.