Subjects in Motion – Subject Movement
If you want to contrast a subject’s movement with its still surroundings, mount the camera on a tripod and choose a shutter speed slow enough to give the desired degree of blur. This depends on how far the subject is from the camera and how fast it’s moving, but you can start off at 1/8 or 1/15 second and see how that looks. Pictures of birds in flight generally require faster shutter speeds and you may need to pan with them to achieve the right balance of sharpness and blur.
Nikon D3, 1/15sec @ f/4 Niall Benvie