The skeletal musculature of different species of birds was analysed using histochemical methods. The innervation characteristics, fibre type distribution and morphometry, and muscle capillarisation were studied in six locomotory muscles of three species of wild birds: the mallard duck, the common coot and the yellow-legged gull. Three fibre types were identified: SO, slow oxidative; FOG, fast oxidative glycolytic; and FG, fast glycolytic. Morphometric findings indicate that the particular oxygen requirements of different muscles matched oxygen supply to mitochondria by reducing fibre cross-sectional area and perimeter, instead of by increasing the number of capillaries around them. The FOG fibres in the flight muscles of the mallard and the coot presented high oxidative activities, short diffusion distances and high capillary densities to cope with the great oxidative demands imposed by flapping flight. This contrasted with the gull, where moderate oxidative capacities, greater fibre sizes and longer diffusion distances where found in response to gliding, a less energy- demanding mode of flight. The ability of mallards and coots to perform considerable amounts of non-steady flapping flight is reflected in the presence of higher proportions of FG fibres. By contrast, the gull flight muscles had low proportions of anaerobic fibres, thus reflecting their reduced ability to fly in tight spaces and as a specialisation for gliding. In the leg muscles of mallards and coots, a greater degree of regionalisation was evident than in those of gulls. This is correlated to the wider range of aquatic and terrestrial locomotory activities that these two species can develop. Due to their highly oxidative characteristics, the anterior part of gastrocnemius and the posterior part of iliotibialis are presumably recruited during sustained exercise and for postural activities. However, the anaerobic profile of the posterior gastrocnemius and anterior iliotibialis, especially in mallards and coots, enable these zones to develop intense activity during short periods of time (aquatic and terrestrial sprints).
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