Have a look through a sigma 150-500mm group on flickr, you'll see this lens can achieve superb results. When it comes to bird photography, it's 99% field knowledge, field-craft, patience and luck. Most of my best photos are taken while out on meets with other more experienced wildlife photographers, because they'll know things I haven't picked up yet and it sort of rubs off on you. One guy who is often on the meets uses a D300 and 80-400VR, a lens which you hear a lot of crap about, especially compared to the 100-400L, but he takes superb shots with it.
Like Rich said, the ~£1k telephotos is an area where canon have a lot more and better choice than nikon (though all the lenses are old designs, they are still superb).
I think the canon super-tele primes are also cheaper (not any more with the mark II canon ones coming out, but those are going to be amazing light weight for what they are) and easier to find on the used market than nikon's, which is a big plus for people looking to drop serious money on a massive super-tele. The one thing nikon doesn't have against canon in the super-tele line is the 200-400VR, canon haven't got anything like it until the 200-400 f/4L 1.4x finaly hits the market and that's supposed to be a brilliant lens for wildlife.
I've seen a lot of wildlife/birds photographs taken with D3's and 200-400's, the combination of a body that handles low light very well and a lens with the perfect focal length range for almost any wildlife (within reason) makes a brilliant tool.
If you're not satisfied with the 150-500 (and you're sure it's the lens not you), maybe have a look at the nikkor 300 f/4? It doesn't have VR which puts some people off, but I see many stunning quality shots from this lens, never heard a user report that isn't stellar.
Unless you particularly want one of canon's unique lenses like the 400mm f/5.6L (a popular choice with birders cos it's light weight, stupidly fast focussing and brilliantly sharp), there's not much to choose between canon and nikon IMO

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