Although I have Nikon, for wildlife I'll recommend you get the Canon 7D.
Pro-build might help out in the dirt and fast burst will help with birds of prey and stuff like that.
Also, take a look at the Canon lenses. For wildlife (especially birds) you'll need from 300mm to 600mm. All the recent telephotos Nikon has in that range are the 300 F/2.8, 400 F/2.8, 500 F/4 and 600 F/4 and they are all above 6,000 $. Canon's range includes some recent lenses that are a little less bright (F/4 or F/5.6) and cheaper, such as a recent 300 F/4, a recent 400 F/5.6 and a 400 F/4 too.
However, there is one thing that is even more important: grab these cameras in your hands, feel them, test them, see if you like the position of the dials and buttons. Even if right now the Canon system is better suited for wildlife, I still prefer Nikon. Canon bodies feel unnatural to me, I have a hard time spinning the dials and finding the buttons. For many people it's the opposite.
Ciao!