Much as the other's have said.
Kayleigh is also right - practice, practice, practice.
I would add - read, read, read. (a book, old threads, web articles).
re: IS
The theorists say turn off IS if on a tripod.
I haven't noticed much difference by doing this (on a 100-400, that was 400 squids cheaper than today when I bought it!).
re: Focus point.
I agree that the focus point is on the nuts which will make the bird soft. Have you read up about 'one shot', 'A1 focus' etc?
Are you using multipoint focus or one spot?
For birds one spot is essential.
re: post-processing
There is a lot that can be done on the computer afterwards.
There is also a large learning curve to this

And it can be overdone - which I know I do.
I wouldn't shell out for a new lens just yet.
IS is great for hand-held shots though.