Adaptations of the flight Muscles in Birds exhibiting differing light characteristics

Abstract

The adaptations of the wing and muscles of flight in several species of birds exhibiting differing flight patterns have been investigated. Sixty two specimen of birds belonging to seventeen species were used. They represented non-flying and flapping fliers. Wing span, aspect ratio, wing loading, wing surface area, body mass and flight muscle mass. were determined. The flight muscles (pectoralis and supracoracoideus)were examined. Muscles on one side of the bird were frozen in liquid nitrogen cooled isopentane and sectioned for histochemistry. The muscles on the other side were fixed in situ through perfusion with buffered 3% glutaraldehyde solution for light microscopy. The histochemical sections were analyzed qualitatively for fibre types and then morphometric ally by point counting method to estimate the volume densities of different types of fibres and capillary densities in various regions of each muscle. The small passerine birds possessed only fast oxidative glycolytic (FOG) fibres in the pectoralis muscle while all the large birds had FOG and fast glycolytic (FG) fibres. In the Domestic Fowl, slow oxidative (SO) fibres were located in a circumscribed region in the caudal part of the proximal one third of the muscle. The supracoracoideus muscle contained only FOG and FG fibres in all species of birds studied. The volume density of FOG fibres increased from the superficial to the deep regions of muscles while FG fibres decreased in the same direction. There were, however, no regional differences in volume densities of individual fibre types along the proximal to distal direction. There was a strong correlation between body mass and flight muscle mass (R2=0.961) and also with wing surface area (R2 =0.926). Non- fliers had higher wing loading than flappers. Aspect ratio did not differ between non-flying and flappers but those birds with intermittent gliding capability had a higher aspect ratio. Fast oxidative glycolytic fibres had a significantly (p < 0.01) higher capillary count per area than FG fibres but individual FG fibres had higher capillaries per fibre than FOG fibres. -xiii- Flapping fliers showed greater adaptations to flight than non-fliers. The high volume density of FOG fibres and capillary counts per area in FG and FOG fibres may enable the agile fliers to meet the extremely high demands of energy required for sustained flight.