If you want to shoot birds, especially the smaller ones are birds of prey at distance, I'd stick with the 7D. Canon have a range of lenses that nikon don't, such as the 400mm F5.6 prime, which is far cheaper than the nikon versions which are faster but have over 3x the price tag. I've owned a D300s and the ISO performance above 800 just isn't good enough for wildlife. The FPS is also crippled when you want to shoot 14bit RAW files, as it slows to 2.5fps even with the grip. The D7000 isn't any better for that either, so unless you want to shoot JPEG to maintain the speed, I wouldn't bother.
Personally I'd look into whats "wrong" with your camera and perhaps send it into fixation to have it cleaned and calibrated with your lenses. Might be something daft like a back/foward focus issue giving you soft images with your lenses.
You'll also need a fair chunk of cash to replace your lenses with the nikon equivalents, making it a very dodgy decision from a financial point of view. I'm a nikon shooter as I much prefer the shape of the body in my hand and how easy it is to use for what I shoot regularly. Canon do things just as well though and you can stare at 100% crops all day if you want, or charts on various websites which argue about how much a few percent really mean in real world performance; or you could just go out with your 7D and actually take photos, practise the art, learn the cameras pros and cons and master it.