Looking at this again, with fresh eyes, your very best shot is the unflashed one, the BG is as good as you'll get IMO in those conditions, the bird's nicely exposed, the BG is not too obtrusive, if want to spice it with some fill, just use TTL with -2 flash compensation at the slow shutter speed you have, probably want second curtain sync in case the bird reacts to the flash. Using a slow shutter speed like that is perfectly valid in low light conditions as long as you have solid support under your lens. In fact this photographer who I've shot with on a number of occasions does not use flash for many shots under the rainforest canopy:


The Pitta and kingfisher shots are unflashed and shutter speed is very low for example:


On your shot a levels tweak to pull in the black and white points improves it quite a bit.