The 500mm IS I had really bad bokeh in certain situations. When shooting in or around grass (birds on the ground) the bokeh would have an awful appearance because of the lack of separation. I don not know what it is about blades of grass but it always seemed to have trouble. The new 500mm II in the same situation isn't as bad.

I wonder if some of what you are seeing is because of the DO build. It is short, compact and light and the bokeh is one of the trade off's.

In the second and third picture in your set the bokeh comes in to play. For my taste the Bokeh doesn't distract. It would have been nice if the back separation was a bit greater but it is a wild bird and you get what you get. To put it in to perspective though, for those two pictures if Bokeh wasn't the topic of discussion I wouldn't have even noticed the bokeh which is the really the idea.

We may be getting in to bokeh peaking, which is probably a rare strain of pixel peeping only much worse.