Bigma 15-500 or 300 IS or 400 no IS

Wild Life and Birding lens options I am considering. Please comment.

Canon 400 f/5.6L (great IQ fast AF)
Weight 2.8 lb

Canon 300 f/4 IS (great IS fast AF)
Weight 2.7 lb (1237 g)

Sigma 150-500mm as it has OS (IS) and HSM (USM).
Decent Fast Auto Focus and Decent Image Stabablization.
Weight 4.2 lbs (1.910kg)

I'll likely want to try a 1.4 or 2x teleconverter on any of these lenses FYI.

-Steve

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-Steve