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....
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.