Depends if you want a zoom - if not I would go for something like the 400mm f5.6 prime, pin sharp and works well with the 1.4tc (but not AF unless you tape the pins or use a cheap Kenko). I find with my zoom that I have it on full zoom for 99% of the time when shooting birds - so why not get a 400 prime in the first place (it is lighter, sharper and cheaper).
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