Sigma 100-300 f/4 Bokeh/Aberration Issue (1 Viewer)

I recently ordered and received a Sigma 100-300 f/4 DG EX to use with my Canon T4i. Focus is quick and accurate, and subjects are sharp. However I've noticed some 'nervous' bokeh in many of the photos that have a background near to the subject. With birds in flight there is not problem, but with photos of birds in the brush (which makes up the majority of my photographs) I see very distracting bokeh and tend to see double of twigs and the like. I would expect the bokeh to be smoother, but I'm not sure if this is natural for the lens or if I got a bad copy. I did purchase it new and it is the newer EX version. I was planning on getting Sigma's 1.4x teleconverter to go with the lens (I've heard good reviews about the pair), but I would want to know if there is a problem with the lens or not first. I attached a few pictures that best show this problem and aren't necessarily great photos (some are taken just for identification). All of the pictures attached were taken at f/5.6 or larger. It hasn't been just a few photos, but the majority of the ones I have taken that have this issue. Does anyone have any ideas? Did I get a bad lens or is this normal? Thanks

BTW I also have a thread in the Critique section with this camera and lens if you'd like to check it out.

I have had a few of the 100-300mm f4 lenses in both Canon and Pentax mount and if possible I always tried to avoid backgrounds that included twigs if I could for this exact reason. It's not just this particular Sigma lens either, I have noticed it when using the 50-500mm OS and to some lesser degree the 70-200mm f2.8.

How did you manage to find a new one? Sigma stopped producing them a few years ago now, I would love to get another in Pentax mount but even old tatty ones command a large premium over the Canon mount lenses due to scarcity and the fact Pentax uses inbody shake reduction.

Was a really nice lens on my Pentax.

first two are not a high enough shutter speed combined with a large aperture ? ,the last one is heat haze /athmospherics IMHO

Mike,
Thanks for your input. I found the lens new on Amazon. It would seem that Pentax mounts are quite rare for this lens from what I've heard about it.

TBF,
The first photo (Fox Sparrow) was taken @ 1/1000 f/5.6, and the Yellow-Rumped Warbler was taken @1/800 f/4.5. I had assumed they were fast enough shutter speeds to shoot handheld (300mm multiplied by 1.6 crop factor would give a minimum shutter speed of ~1/480). I have shot only in Manual, so are my settings wrong? I've just found this problem very distracting in some photos.

Here's a few recent photos;
1. 1/640 f/5.6 ISO 1600
2. 1/640 f/5.6 ISO 1600
3. 1/800 f/5.6 ISO 250
The bokeh seems a bit better the farther away the background is (as it should be), but in other shots I'm still getting the distracting bokeh.