In this study we present a preliminary analysis of the structure of the complex multisyllabic warble song of the budgerigar Melopsittacus undulatus, a small, flock-living parakeet. We recorded eight male budgerigars from three different social groups, and one female. Budgerigar warble songs consist in long sequences of syllables that are diverse in structure, ranging from simple clicks to multinote, frequency-modulated musical-sounding syllables. We identified 42 syllable classes. Males shared a significantly greater percentage of their warble syllable-class repertoire with males in the same social group than with males in different groups. One male budgerigar preferentially imitated the syllables and temporal pattern of the abnormal warble of his cagemate (a male bird that had been reared in acoustic and social isolation) rather than the normal warble of other male budgerigars in adjoining cages. We discuss vocal imitation of social companions as a process of vocal learning, the potential role of warble song in individual and group recognition, and implications for syntactical structure of budgerigar warble song.