When the birds are way too far ie very small in the frame, the AF point covers a very large area. So one is not sure as to where the camera focuses. In such situations the image would be soft. In flight photography, track the bird before it flies past. You have to allow time for the AF point to acquire focus ie lock on to the bird and then move the camera and when the bird is in the region where you want it to be, just click the shutter.