https://www.selleckchem.com/products/nsc16168.html The structural similarities with the references are 0.9922 for the complete FOV and above 0.9999 for the ROI.Bird feathers are complex structures that passively deflect as they interact with air to produce aerodynamic force. Newtonian theory suggests that feathers should be stiff to effectively utilize this force. Observations of flying birds indicate that feathers respond to aerodynamic loading via spanwise bending, twisting, and sweeping. These deflections are hypothesized to optimize flight performance, but this has not yet been tested. We measured deflection of isolated feathers in a wind tunnel to explore how flexibility altered aerodynamic forces in emulated gliding flight. Using primary feathers from seven raptors and a rigid airfoil, we quantified bending, sweep, and twisting, as well as ∝ (attack angle) and slip angle. We predicted that 1) feathers would deflect under aerodynamic load, 2) bending would result in lateral redirection of force, 3) twisting would alter spanwise ∝ "washout" and delay the onset of stall, and 4) flexural stiffness of feathers would exhibit positive allometry. The first three predictions were supported by our results, but not the fourth. We found that bending resulted in the generation of lateral forces towards the base of the feather on the order of ~10% of total lift. In comparison to the airfoil which stalled at ∝=13.5°, all feathers continued to increase lift production with increasing angle of attack to the limit of our range of measurements (α=27.5°). We observed that feather stiffness exhibited positive allometry (∝ mass1.1±0.3), approximately consistent with previous research showing that stiffness does not scale as predicted by geometric similarity (∝ mass1.67). These findings demonstrate that feather flexibility may provide passive roll stability and delay stall by twisting to reduce local ∝ at the feather tip. Our findings are the first to measure forces due to feather d