Respiratory contributions to birdsong-evolutionary considerations and open questions.

Respiration plays a central role in avian vocal behaviour by providing the airstream that induces vibration of vocal folds. In this role, respiratory movements dictate the coarse temporal pattern of song, while simultaneously fulfilling its vital functions. Whereas these aspects have been investigated in oscines, little information exists in other taxa. Broad taxonomic information is, however, necessary for addressing questions regarding evolutionary specializations of the respiratory system. Acoustic recordings of unstudied taxa suggest that rapid action by respiratory muscles is a basal trait within birds. In addition to controlling the timing of vocalization, respiratory activity also influences acoustic features such as sound amplitude and frequency. The latter is more strongly influenced by respiratory driving pressure in non-vocal learners. Singing, as a highly dynamic respiratory activity presents an opportunity for studying detailed ventilation patterns and thus could give insight into the basic control of airflow in the avian lung-air sac system. Although we have learned many details of how respiratory control is tied into cortical song control, many open questions remain. Control of respiratory pacemaker circuitry by upstream vocal control centres, respiratory input in initiation of vocalization and the use of online feedback from the respiratory system are all incompletely understood.This article is part of the theme issue 'The biology of the avian respiratory system'.