THE EFFECTS OF THE WINGBEAT CYCLE ON RESPIRATION IN BLACK-BILLED MAGPIES (PICA-PICA)

Interclavicular and posterior thoracic airsac pressures, tracheal airf lows and pectoralis muscle activity were recorded simultaneously to de termine the effect of the wingbeat cycle upon the function of the resp iratory system. The effects of the wingbeat cycle on the relative posi tions of thoraco-abdominal skeletal structures were also assessed usin g high-speed X-ray cinematography of magpies Pica pica flying in a win dtunnel. We found that the furcula bends laterally on the downstroke a nd recoils medially on the upstroke, as previously described for starl ings, and that the coracosternal joint (the most consistently visible point on the sternum for digitization) is displaced dorsally during th e downstroke and ventrally, with respect to the vertebral column, duri ng 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 occu rs during expiration its expansive effect can reduce the expiratory su pra-atmospheric airsac pressure by 63 % (0.23 kPa). Corresponding chan ges occur in tracheal airflow. Changes in respiratory parameters durin g short flights with respect to resting values include a doubling of t idal 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 betwe en wingbeat and respiratory cycles that may result in minimal interfer ence and maximal assistance from the wingbeat upon the respiratory cyc le.