I'm afraid I was not paying a lot of attention to the F stop at all - just out bird hunting - I'm new to this.  Should try to shoot with the lowest F number I can I suppose, and still get a fast enough shutter speed.  
Am I further ahead to set a speed and let the camera pick the F stop?  Say set it at 1/100 or 1/200 and see what it give me?  How slow can I go and get a descent shot away.  Suppose that really depends on whether the bird is still or flying eh?  
For a sitting bird what is an acceptable shutter speed?  Or should I us aperture priority and dial in F/8 or even F/5.6 which this lens will do throughout the full focal range?  
I'll look through my shoot and see if I did get a shot with lower F number.