The macroscopical structure of the organ of voice in songbirds has long been known, but detailed information on the microscopical anatomy of the syrinx has generally been lacking. Observations based largely on macroscopical evidence have led to a number of erroneous interpretations of function of various syringeal components, and lacking microscopical information, the vocal mechanism of birds cannot be adequately understood.

A wide variety of passeriform bird syrinxes have been studied by means of serial sections. Although there is much less variation in syringeal anatomy amongst songbirds than there is in the other orders of birds, and although all songbird syrinxes conform to the same basic pattern, there is nevertheless marked variation in various syringeal components between different passerine groups. Variations in syringeal structure within families Corvidae (Corvus corone, C. frugilegus), Sturnidae (Sturnus vulgaris, Gracula religiosa), Turdidae (Turdus merula, Erithacus rubecula), Hirundinidae (Delichon urbica), Ploceidae (Passer domesticus) and Paridae (Parus major, Aegithalos caudatus) are described and discussed. The significance of these findings in relation to bird sound production is discussed.