The effects of the wingbeat cycle on respiration in black-billed magpies (Pica pica)

Interclavicular and posterior thoracic airsac pressures, tracheal airflows and pectoralis muscle activity were recorded simultaneously to determine the effect of the wingbeat cycle upon the function of the respiratory system. The effects of the wingbeat cycle on the relative positions of thoraco-abdominal skeletal structures were also assessed using high-speed X-ray cinematography of magpies Pica pica flying in a windtunnel. We found that the furcula bends laterally on the downstroke and recoils medially on the upstroke, as previously described for starlings, and that the coraco-sternal joint (the most consistently visible point on the sternum for digitization) is displaced dorsally during the downstroke and ventrally, with respect to the vertebral column, during the upstroke. In magpies, there are generally three wingbeat cycles during a respiratory cycle. When downstroke occurs during inspiration, its compressive effect reduces the inspiratory subatmospheric airsac pressure by an average of 92 % (0.35 kPa), whereas when upstroke occurs during expiration its expansive effect can reduce the expiratory supra-atmospheric airsac pressure by 63 % (0.23 kPa). Corresponding changes occur in tracheal airflow. Changes in respiratory parameters during short flights with respect to resting values include a doubling of tidal volume and a tripling of respiratory frequency. We conclude that the wingbeat cycle can have a substantial impact on respiratory system function in the flying magpie, and that this represents a mechanical basis for breathing patterns and patterns of phasic coordination between wingbeat and respiratory cycles that may result in minimal interference and maximal assistance from the wingbeat upon the respiratory cycle.