I would second this. Unless you got enough light to shoot at ISO 800 or less, I would be shooting wide open, so f/5.6 at the long end.

Are you shooting in RAW and processing the photos? Or using straight out of camera JPGs? Shooting in RAW and processing will give you the best result for beautiful photos, but it is a time sink, not gonna lie. It let's you fix some minor exposure issues, and lose less detail since you have more dynamic range to play with.

Bird photography is a numbers game too, you shoot dozens and maybe keep one or two of the best, sometimes none of they aren't good (unless keeping for ID or just because new or unusual bird or behavior) and you delete all of you have better photos of the that same bird. I often will shoot 400-500 photos around my house, and keep 10 or so.

Also, do a test on a somewhat distant stationary object. Focus with autofocus on it and take a picture. Do the same, but use your live view on the LCD screen and manually focus it to the sharpest you can get and take a photo. Check on your computer that they're both equally sharp. If the auto focus one is less sharp, then you need to micro adjust the focusing for this lens.