Inspiratory muscle activity during bird song

The apparently continuous flow of bird song is in reality punctuated by brief periods of silence during which there are short inspirations called minibreaths. To determine whether these minibreaths are accompanied, and thus perhaps caused, by activity in inspiratory muscles, electromyographic (EMG) activity was recorded in M. scalenus in zebra finches and in M. scalenus and Mm. levatores costarum in cowbirds, together with EMGs from the abdominal expiratory muscles, air sac pressure and tracheal airflow. EMG activity in Mm. scalenus and levatores costarum consistently preceded the onset of negative air sac pressure by approximately 11 ms during both quiet respiration and singing in both species. The electrical activity of these two muscles was very similar. Compared with during quiet respiration, the amplitude of inspiratory muscle EMG during singing was increased between five- and 12-fold and its duration was decreased from >200 ms to on average 41 ms during minibreaths, again for both species, but inspiratory muscle activity did not overlap with that of the expiratory muscles. Thus, there was no indication that the inspiratory muscles acted either to shorten the duration of expiration or to reduce the expiratory effort as might occur if both expiratory and inspiratory muscles were simultaneously active. Inspiratory and expiratory muscle activities were highly stereotyped during song to the extent that together, they defined the temporal pattern of the songs and song types of individual birds.