With a 70-200 and a 2x converter you would need to be within 10-15 ft for a decent sized shot of a small bird so your talking of using a hide or a blind. It would also be necessary to raise the ISO for shots of small birds like robins. I have produced many successful shots at a speed of 1/60th but things have to be timed when the bird is inactive. Humming birds well I've never seen one here in Lincolnshire but I would imagine their bodies are still at times whilst feeding but their wings are going like hell! Depends what results your after. With a long lens like the Bigma a lot of people use flash extenders in poor light, perhaps that may be worth a look? Used a Bigma for years with a 1.4 converter but personally I set up somewhere that gives me the conditions I want using a blind. Flash I found disturbed the wildlife and their welfare is paramount. Butterflies and dragonflies look much better,imo, in natural light as does all nature.