I’ve long been a proponent of using the same brand of lenses as your camera body (or bodies), for both practical and aesthetic reasons. But there are limits to this philosophy including availability of the lens you need for a specific task, and budget. The latter has become a much bigger factor due to the recent large price increases on lenses caused by currency values and market forces. Especially with new long lenses—supertelephotos in excess of 400mm—cost has become all but prohibitive.
If you want to make good images of birds and small mammals on a regular basis you need a fairly long focal length lens; 500mm is generally considered a reasonable threshold. 600mm and 800mm lenses make closing that distance gap even easier, but again the cost goes up commensurately. So what might be some reasonable alternatives?
One is Sigma’s 50-500mm f4.5~6.3 APO DG OS HSM (how’s that for a name!) in either Canon or Nikon mount. Not only is this lens a relative bargain (currently on sale too), but it gets high marks by some very respected shooters (see Arthur Morris’ Birds as Art Bulletin #426). It’s not reasonable to expect a sub-$2000 lens to perform exactly like a $10,000-plus lens, but the 500mm focal length—even at a reduced maximum aperture—might make the difference between getting certain shots or not getting any at all.