I think it is impossible not to be disappointed first time you use a long lens. There is so many issues that can affect the image quality.
I totally agree with the other advises, but I would like to add a few more issues.
The first is the atmospheric conditions. If the sun is high or if there is wind, the air movements can ruin sharpness. Try to avoid those conditions. And ad try to get as close as possible – learn to avoid that the birds are afraid of you. Takes a lot of practising.
It looks like you are shooting jpg with active D-lightning on. It can be great for snapshots, but what it does is to compensate for underexposed areas, introducing more noise. If you are shooting jpg, and expose correct, you can get the best possible result in post processing.
The last thing I will mention is the choice of camera. I have found that the autofocus function of the D7000 is somewhat difficult, especially if the subject is relatively small in the image and if the contrasts are low. I afraid I don’t find the AF reliable for use with long lenses. I’m not sure what the problem is, and there has been a lot of debate about it. If you want to use DX, I don’t think there is a better camera right now than the D300/D300S, but it shows its age with respect to noise and sensitivity. We really need a new pro grade DX!