I have started to use the 1.5 converter more than the 2X cause I am happier with the image quality, obviously if I try not to use any converter. Took some of a pair of Dartfords on all three today and thought the best was when using the 1.5X converter.
Image not cropped at all just resized for posting, taken from about 20ft.
Your Dartford Warbler shot is possibly the best argument I have seen for this camera from a birder's point of view though I suspect there was a lot more that went into getting the shot than just the camera - I don't think I could get to within 20' of a bird like that.
Mulling lots of ideas at the moment, the new sigma 120-300, 400 f5.6, a canon 1.4x extender onto my 300 f4 and kenko 1.4x. Then I saw Roy Churchill's swallow on a post shot and thought some more. Not rushing out and buying anything at the moment, the wife has just had a back op so won't be going anywhere very much for the next 3 months, but looking for bang for buck, same as we all do, but for the budget I have set aside for the sigma I could buy all the rest - and possibly the 70D when it finally arrives.
Thanks to all the contributors to this thread, it's one of my daily reads now