Flocking is the natural social structure of the budgerigar Melopsittacus undulatus. Its repertoire is well known and includes a number of acoustically and functionally distinct calls (contact call, alarm call, etc.). The social context plays a large role in the learning of these calls. Our aim in this study was to determine if the social context (presence or lack of a partner) may modulate the significance attached to a signal and thus modify the behavioural responses to a conspecific call. To investigate this, unmated paired birds and temporarily isolated ones were submitted to the playback of a specific alarm call. The tested sessions were videotaped and then analysed frame by frame. Multiple qualitative response measures were used and quantified. They included both vocalisations of subjects in response to the sound playback and a graded series of postures, ranging from a variation of orientation relative to the sound speaker location, up to alert behaviour and flight. Our results clearly indicate a behavioural plasticity of budgerigars according to the social context. The most prominent difference between isolated and paired birds is that the former are significantly more likely to give an acoustic response while the latter show an increased motricity and even an escape flight. Isolated birds give repeated calls in response to the playback signal, as they would maintain contact with the flock. At this level, paired birds are quite unresponsive. The modulation of the behavioural responses observed in this species indicates that the meaning of a signal is related to the social context.