I agree with Neil - post some samples. I would only use AF Fine Tuning if you are absolutely sure that is the issue and your gear does not need repair.
Try making it easier on yourself. Stop photographing birds and photograph some easy subjects or targets at a distance of 10-15 feet. Use a shutter speed of 1/500 or faster. Use the center sensor for AF, a tripod, mirror up, and cable release. Defocus each time and try at least 10 images. If you are still getting soft images, explore a problem with your gear. If not, it's probably technique.