A Morphometric Study On The Dynamic Structure Of Primary Feathers In Birds

Primary feathers are dynamic organs for bird flight. Study of their structure is essential for discovering the flight adaptation in birds. In our previous studies, a tube-liked and U-shaped structure were identified from primary feathers as a critical facility for generating thrust power during down stroke, and was named as the Nozzle. In order to further address the role of nozzle in flight mode, attitude, and residential habitat, we carried out a morphometric study on the structure of nozzles in 34 bird species of 13 families,8 orders with variable body masses and flight modes, including gliding, flapping-gliding and flapping. Results indicated that the nozzles were normally quadrilateral in cross section, falling into 3 types due to the close extent, namely U-shaped nozzle, semi-closed nozzle and closed nozzle. We used body mass as criteria to test the effectiveness-related morphometric indexes of the nozzle, and found the width (measured as the inter-barb space) varied greatly among bird species and was not significantly correlated with body mass (R2=0.014, p=0.498,α=0.05). The height of nozzle (measured as the height of barb) was significantly correlated with body mass (R2=0.865, p=0.000,α=0.05), so was the cross section area of nozzle (R2=0.761, p=0.000,α=0.05). These results suggested the power requirement depends on body mass, and birds improve the effectiveness of nozzle through extending the internal diameter. We also calculated the ratio of width, height and cross section area of nozzle to body mass. The results indicated the three ratios were all significantly smaller for birds requiring run-ups for take-off than those not requiring run-ups, suggesting birds would use behavioral compensation when the effectiveness of nozzle are not sufficient relative to take-off weight. In open habitats, birds are able to use run-ups to compensate the dynamics for take-off, while in closed habitats, birds are restricted by space for run-up, thus would reduce body mass as the power of nozzle has a upper limit despite how morphometric indexes are optimized. The direction of nozzle measured as the angle between rachis and barbs was not related to take-off capability (R2=0.003, p=0.758), but play an essential role in ajusting attitude of flight.