[Charlotte]

I've been looking at teleconverters as well - for Nikon, but my comment should apply to Canon/other as well (I think) ... and have settled on getting a 1.7x whereby you lose 1.5 stops (a happy medium I think) and the Nikon one is very compact + lightweight compared to the 2x one; not sure if this relates the same way in the Canon world though, but if it does, you could use two of these for a really good reach (but lose 3 stops of light).

Losing AF could be tricky for wildlife shots, especially birds, which you say is your 'thing', so that might be a strong consideration for you if you use a teleconverter with the lens you mentioned.