Male singing behaviour correlates with extra–pair success in several passerine birds. Singing interactions during territorial contests provide relative information on the males involved. Such information may be important in female extra–pair behaviour and eavesdropping on singing interactions among males may allow females to make such relative assessments. We used interactive playback to instigate singing contests with male great tits during the peak fertile period of their mate in an attempt to alter females ‘assessment of mates’ quality relative to neighbours (potential extra–pair partners). We escalated a contest to one male (by overlapping his songs) and then subsequently de–escalated a contest (by alternating) to a neighbour. Intrusions onto neighbouring territories by females mated to either treatment male were then monitored. Females mated to escalation treatment males were more likely to intrude following playbacks than females mated to de–escalation treatment males. Although the absolute song output of males did not differ between treatments, males produced more song relative to playback in de–escalation treatments and relative song output was positively correlated with female intrusions. Therefore, female great tits eavesdrop on singing interactions and change their visitation rates to neighbouring territories according to their mate's singing performance relative to neighbours.