I heard, and also what I have experienced with my own lenses, that for any lenses, stopped down 2 full apertures, would give you the sharpest image a lens capable to produce. Say, my Canon F/4L lenses, when stopped down to around F6 the images start to get sharper. Also from many Lenses review websites, I heard, for most lenses, F6-F11 produce the sharpest images. F8 normally the sharppest point. you get the image. When photoing fast moving objects, aka flying birds, a sunny day is really needed. this way, you can reduce F down to say, F8, increase the shutter speed to 1/1250 or faster, while you are still able to get enough light into your lens to produce a sharp image.
above is what I have heard and also matches my own experience. Hope it can be helpful.