The singing and related vocalizing of great crested flycatchers, Myiarchus crinitus (Aves, Tyrannidae), included at least 12 different vocalizations. Each vocalization correlated with (and thus made information available about) a distinctive range of behaviour, from unassertive to more actively responsive, provocative or even confrontational. Four were used by individuals primarily when alone and not initiating close social encounters. The eight other vocalizations were uttered primarily in close encounters: one in preliminaries to such events, two predominantly in encounters with opponents, another in close associating of mates, two in duetting, and two infrequently, mostly in confrontations. The division of the repertoire into sets of vocalizations providing information about either low or elevated probabilities that a signaller will undertake the behavioural initiatives that precede, shape or sustain social interactions parallels a division found in other recently studied passerines. Such information is pertinent to individuals' decisions to forego or start close interactions with, for instance, competitors or mates. The information should be useful to a great many species, not just birds, but also mammals and various other animals — species in which individuals keep each other informed by singing when they are apart.