Abstract: Flight feathers of birds interact with the air flow during flight. How the observed low drag and high lift values at wind speeds from 9.0 to 19.8 m/s can be achieved due to the feather aerodynamics remains unknown. In the present paper, we tested and compared morphological changes, drag reduction and flow visualization results of intact, damaged, and artificial feathers at different wind speeds in a wind tunnel. Through the analysis of the drag force and resultant force angle, we proved that the integrity of feathers, whose barbs are usually closely interconnected, played an important role in the drag, which potentially triggers excellent drag reduction performance. The wind tunnel tests indicated that intact secondary feathers had a surprisingly high maximum drag reduction property at v?=?9 m/s compared with the feathers, where the integrity of barbs was damaged. The hook cascades facilitated elasticity under pressure and suitable permeability in an intact feather, when the hooks were interlocked. It was indicated that the suitable permeability of intact feathers would prevent flow separation and reduce drag force at low wind speed; at high wind speed, elasticity under pressure and suitable permeability in an intact feather would facilitate strong squeezing effect, helping feathers withstand larger aerodynamic forces to which they might be subjected during flight. It was revealed that the intact secondary feather is a compromise between strong lift generation and drag reduction, which has a great significance for the bird’s flight.

Key words: Morphological integrity , · Secondary feather , · Drag reduction , · Wind tunnel , · Smoke visualization , · Bioinspiration