Fig. 4. Respiratory changes cause fluctuations in oxygen consumption. (A) The pattern of oxygen consumption (_O2) and respiration (air sac pressure, P) can vary substantially and rapidly. The horizontal line in the pressure trace represents ambient pressure. (B,C) The volume and duration of each expiratory and inspiratory pressure pulse were calculated along with the mean oxygen consumption rate (_O2) for that segment. Data points for expiration and inspiration are values calculated by dividing the volume of each pulse by its corresponding duration, giving an estimate of respiratory rate and effort. Whereas in B the axes for expiratory and inspiratory effort are scaled to illustrate small fluctuations unrelated to song, C shows these data at full scale. These traces illustrate that oxygen consumption reflects even small fluctuations in respiratory activity. As respiratory activity increases (marked by first arrow in A), demonstrated by shorter air sac pressure pulses of higher amplitude, _O2 also increases. Shortly before song (second arrow in A) there is a small decrease in oxygen consumption that corresponds to a decrease in respiratory rate (long duration and small amplitude pressure pulses). Respiration increases during song followed by a distinct peak in _O2. After song there is a significant decrease in respiration (pulses of short amplitude and duration) and a large decrease in oxygen consumption. Note that vocalizations are characterized by markedly increased amplitude of air sac pressure, as illustrated by the song bout, but also by several calls around the 10 s and 50 s mark (A). Because the syringeal resistance changes for vocalization and airflow patterns change, this altered pressure is not accompanied by a correspondingly large change in oxygen consumption.

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