A Preliminary Study On The Microstructure Of Bird Primaries And Their Significance For Flight

Remiges are organs playing critical role in flapping flight of birds. Their asymmetry and morphogeometry and their aerodynamic functions, as well as structure and mechanical properties of rachis medullar and have been studied. However, these findings are still not able to fully explain the extreme flight ability of bird. We here observed microstructure and measured corresponding parameters of first primaries in 29 species of 9 orders that are varying in body mass and flight performance. Results indicated:1. The trailing edge vane of primary feathers formed by barbules is a semi-transmitting structure that distal barbules have relatively large angle to the plane of vane at basal part so as to allow air transmit during upstroke, and proximal barbules distorted to achieve a small angle to the vane plane at basal part and arranged in a overlaying pattern so as to prevent air from transiting during downstroke.-Such construction of vane largely reduces energy cost during upstroke and yield high lift and thrust during downstroke.2. The barbs curl at ventral area in medium and large bird. The curliness forms a open-semi-open or closed and gradual contracting nozzle. Air may enter the nozzle through basal opens and turn directing to trailing edge and be accelerated. A strong thrust will be yield when accelerated air spurt out of the nozzle. This mechanism is critically important for takeoff and landing of bird as well as maintaining in the air.3. The inner surface of the contracting nozzles is not smoothing and yields friction during air passes through. The friction impacts the moment of air spurt by trading-off between length and diameter. Different species adopt different length-diameter nozzle and closing extent to meet different demand of thrust and lift.4. Foam texture of rachis is a structure distributing load to the whole rachis and a source of elasticity. It is a key strategy to reduce weight of rachis while maintain mechanical performances.