Main forces on a flying bird are lift, weight, thrust, and drag. Quantitative visualization of the flow shows how these forces result from the interactions between bird and air. Conventional flow around cambered arm wings with rounded leading edges and sharp trailing edges is attached and deflected downwards behind the wings. A starting vortex needs to be shed before the full lift force is produced. Hand wings with sharp leading edges are flat and often used in swept back position to induce leading edge vortices (LEVs) above the wing. LEVs produce lift and drag instantaneously. The aerodynamics of flapping flight is more complex than that of gliding flight because thrust needs to be generated as well as lift. Birds most probably use combined effects of attached and LEV flow to accelerate air downwards and backwards. Most tails operate as delta wings when spread. LEVs are probably the main aerodynamic mechanism.
Keywords: lift, weight, thrust, drag, flow visualization, conventional flow, leading edge vortex, flapping flight, gliding, delta wings