Many of the signals used for communication by animals are capable of transmission over long distances, that is, several times the usual spacing between individuals. From this observation it follows that a number of signallers and receivers are within communication range of each other and have the potential to interact. Such a grouping of several signallers and receivers is referred to as a communication network. They are thought to be common in all signalling modalities. The concept of communication networks has important implications for the evolution of signalling systems and the perceptual abilities of receivers. Studying communication networks presents a number of challenges, not least of which is how the signals of the individuals in a network can be simultaneously monitored without affecting the animals' behaviour. I describe a passive monitoring system for acoustic signals. This acoustic location system (ALS) uses the differences in arrival time of the same sound at several microphones to calculate its site of production and simultaneously record the signals of interest. This technique and its potential are explained using examples of male song birds (great tits Parus major) defending territories. In particular, the timing of singing interactions between neighbouring territory holders and their use of song variants will be related to location with respect to each other and shared territory boundaries. Preliminary indications are that timing and song variation are used to "address'' an otherwise widely broadcast signal to a particular member of the communication network.
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