I have the XT-1.
It is brilliant, light and an ease to use. I use it for close-ups, butterflies, plants and general shots, and it is very good, I can't fault it. It's expensive along with the lenses, it can't match the big Nikon, nor should it.
If you own Oly lenses, then I would go down that brand route. I would steer clear of Nikon's mirror offerings,
For bird photography you need to be close, but I'm fussy and images have to sharp to be acceptable. The Fuji will give you these but at the moment I don't have the 400mm lens, so I'm a bit limited. The new software updates should improve AF.
I'd be keen to see just what results can be obtained from the new 400mm lens.