paul goode said:Hi Graham,
I've pulled one line out of your original post:
'who lives in dark and dismal (at least during the winter months) Sweden.'
Unless you want a summer only hobby I think that a body that can handle high ISO noise is essential and, quite simply, thats the Canon. Matching that with a lens with a USM/HSM motor should give you the best chance of keeping shooting throught the winter. Lens choice is personal but go for the fast AF provided by those motors.
I totally agree with this, the high ISO performance of the Canons is simply amazing. In you situation I'd go for the Canon with either the 100-400IS or the 400 f5.6... hard to know which one - from what I've seen the prime handles a tc better (good if you ever wanted a bit more reach) and has slightly faster AF. However the zoom is a bit more versatile, has better close focus and has IS... As you are used to using a camera with a zoom lens, you might find the prime a bit limiting.
Here's a link to an ISO1600 shot taken with a Canon 350D (sorry I know I've linked it before...) http://www.birdforum.net/pp_gallery/showphoto.php/photo/77940