Body size, pectoralis composition, aspect ratio of the wing, and forward speed affect the use of intermittent flight in birds. During intermittent non-flapping phases, birds extend their wings and glide or flex their wings and bound. The pectoralis muscle is active during glides but not during bounds; activity in other primary flight muscles is variable. Mechanical power, altitude, and velocity vary among wingbeats in flapping phases; associated with this variation are changes in neuromuscular recruitment, wingbeat frequency, amplitude, and gait. Species of intermediate body mass (35–158 g) tend to flap-glide at slower speeds and flap-bound at faster speeds, regardless of the aspect ratio of their wings. Such behavior may reduce mechanical power output relative to continuous flapping. Smaller species (<20 g) with wings of low aspect ratio may flap-bound at all speeds, yet existing models do not predict an aerodynamic advantage for the flight style at slow speeds. The behavior of these species appears to be due to wing shape rather than pectoralis physiology. As body size increases among species, percent time spent flapping increases, and birds much larger than 300 g do not flap-bound. This pattern may be explained by adverse scaling of mass-specific power or lift per unit power output available from flight muscles. The size limit for the ability to bound intermittently may be offset somewhat by the scaling of pectoralis composition. The percentage of time spent flapping during intermittent flight also varies according to flight speed.