Modulation of flight muscle power output in budgerigars Melopsittacus undulatus and zebra finches Taeniopygia guttata: in vitro muscle performance.

The pectoralis muscles are the main source of mechanical power for avian flight. The power output of these muscles must be modulated to meet the changing power requirements of flight across a range of speeds. This can be achieved at the muscle level by manipulation of strain trajectory and recruitment patterns, and/or by intermittent flight strategies. We have measured the in vitro power outputs of pectoralis muscle fascicles from budgerigars Melopsittacus undulatus and zebra finches Taeniopygia guttata under conditions replicating those previously measured in vivo during flight. This has allowed us to quantify the extent to which different power modulation mechanisms control flight muscle power output. Intermittent flight behaviour is a more important determinant of flight power in zebra finches than budgerigars. This behaviour accounts for 25-62% of power modulation relative to the maximum available mechanical power output in zebra finch, compared to 0-38% in budgerigars. Muscle level changes in fascicle strain trajectory and motor unit recruitment, rather than intermittent flight behaviours, are the main determinants of pectoralis muscle power output in budgerigars at all speeds, and in zebra finch at speeds below 14 m s(-1).