I guess there wouldn't be a need for a zoom at a fixed, pre-focused scenario, no.
But you mentioned "Shooting prefocused birds flying from close by with a 80-40mm lens" (I assume you meant 80-400mm), and the research I have done suggests that the Nikon 80-400 is only tolerable up to about 200 mm, and after that it performs mediocre at best. By contrast, the Canon 100-400 mm is considered still very strong (or at least acceptable) at the long-end of its reach. Since you also asked, "Is there any agreement on Canon versus Nikon for wildlife shooting or just flame wars?", I figured this (80-400mm zoom) was the tool range you were inquiring about.
Finally, Michael just did a resolution/print comparison on the 50D, 5D, 5DMkII, and 1DMkIII ... and the quality of the 50D prints were indistinguish from the higher-end Canons, up to 13x19," which was another point of consideration you mentioned. Only at larger print sizes did the 50D begin to fall behind, but up to 13"x19" its printed images were considered "professional quality" right alongside the big guns.
Hope this helped,
Jack