Interactive playback was used to test observational findings that different vocalizations uttered by singing great crested flycatchers, Myiarchus crinitus (Aves, Tyrannidae), each provide distinctive information about behavior. We present the results of these tests and interpret their significance in combination with the observations. The following predictions were confirmed: 1. Few weep or weeuh songs, and no thrr, were uttered by subjects that approached playback; 2. Churr, common during observations of more active and changing behavior, often predominated during subjects' initial approaches and searches for simulated intruders; 3. Churr was succeeded by wit, which had been found during confrontational behavior, and weihp, which came with movement near opponents or playback; and 4. Quick answers to vocalizations of mates, opponents, and playback were with rreet, also uttered during attack behavior. Functionally, birds uttering the unassertive vocalizations (weep, weeuh, and thrr) may have been taking only minimal initiative to interact, and simply advertising their presence and potential responsiveness. With increasing numbers of churr, subjects maintained social contact with mates or opponents or probed for responses from quieter birds. Wit and weihp may strongly provoke opponents to respond. Further escalation involved met, weed, and wi. In contrast, ch-ee was a defensive threat. This species sings with more vocalizations than do other tyrannids we have studied. Its vocalizations correlate with relatively fine distinctions among behavioral categories. Yet the vocalizations, like those of the other species, provide information about different extents of initiative that a singer will show in interacting. Such information could be fundamentally important in shaping and stabilizing social relationships, not just of tyrannids but also of many other kinds of animals that use singing to interact with one another while at a distance.