Bird song is capable of transmission over distances of several territories. Therefore signallers and receivers constitute a communication network. Eavesdropping is a type of receiving behaviour that is only possible in communication networks; it is defined as extracting information from interaction between other individuals. Such receiver behaviour has the potential to provide information on relative competitive ability. In the context of territory defence and bird song this means that an individual can, by eavesdropping, use its neighbours as yardsticks against which to judge unknown intruders before having to interact with those intruders. We used interactive playback with male great tits Parus major to simulate a singing intrusion with a subject's neighbour. We then simulated an intrusion with the subject. Interactive playback was used because it allows the playback stimulus to be varied song by song, and therefore we could behave as different types of intruders. We signalled willingness to escalate by overlapping the neighbour's songs and increasing strophe length relative to the neighbour (song type was matched). We signalled unwillingness to escalate by alternating playback and reducing playback strophe length (again, song type was matched). If subjects had extracted information from the interaction between their neighbour and playback (i.e. if eavesdropping had occurred) we would expect the subject to respond differently to the intruder depending on its apparent willingness to escalate. The first results from such experiments (6 males) show just such a difference. When subjects were subsequently presented with "neutral'' interactive playback (same song type as used by the intruder, alternating pattern of singing and matching strophe length) they responded by overlapping song and remaining at some distance to intruders which had signalled willingness to escalate with their neighbours. In contrast, they responded by quickly approaching closely to intruders which had apparently been unwilling to escalate with their neighbours. This difference in response is best interpreted as subjects using information on the fighting ability of the intruder to modify the level of their aggressive response. They rapidly escalated to close range territory defence with apparently weak opponents, but responded more cautiously to apparently strong opponents. These results are consistent with eavesdropping and support our contention that such behaviour will be an important feature of receiving in a communication network.