Hi Izzie, you are getting them fairly well. There will always be a high reject rate with birds in flight. As you get better you would think the reject rate would reduce but unfortunately it never seems to because we get just fussier....
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Actually I think all photography is like that - the better we get the more difficult it is to be happy with the result.
What sharpening are you doing? Some are possibly a bit soft because of focus or movement but I think all of them could be sharpened more successfully. On dull days try ISO 800 so you can stop down a fraction and increase the shutter speed. I know the D300s will have a little bit more noise at ISO 800 but I think the benefit in sharpness will more than compensate. Remember that when you reduce the size for posting you also need to adjust the sharpening.
I have tweaked #4 a bit and will post it if you wish.