www.luminous-landscape.com has good reviews of the better Canon glass, including a 400 5.6 lens that might be appropriate (sharp enough that it should surely be OK with a converter too); check out
www.naturfotograf.com for Nikon reviews. The latter site also has useful information for economically retrofitting older (i.e. cheaper) manual focus Nikon glass to work with all the metering functions and exposure modes of the latest cameras.
Consider focusing. If you go (relatively) inexpensive and lightweight with something like the Canon EF 400 5.6 as opposed to a big f2.8 or f4 telephoto, if you then also stick an f5.6 lens on a converter, you then run afoul of the f5.6 limit on autofocus with most consumer and mid-range cameras (the Canon EOS 1D series and Nikon D1 series as well as the top film bodies do better than that).
Consider what the previous poster said about customer needs. Also consider how close you can get to what you're shooting, and practical details such as how much of the frame your client needs you to fill with the actual bird for certain shots as opposed to its overall setting.