Gordon thanks...... just love the Bigma..... but I did have a few issues when first purchased. Let me just say, for the money, the Bigma has got to be great value. Now back to the issues, which were not a lot, as I tend to shoot wide open a lot, pre D300, back focus came into play, D2Xs was re calibrated, minor adjustment, re test lens same issue, send lens in for re calibration and it came back with the report of update chip, firmware, and calibrate. Most of what I shoot is in close so any softness will show, more so if you use a 1.4 TC or 2X TC, although the Bigma then becomes manual focus. With out the TC's the Bigma does focus quite quickly and a joy to use
I do not shoot as much as I used to, but do like to have my gear working well and sharp.
Had a look at your birds pixs, which are good, but I would send the lens back for a calibration as it does look a tad soft. I always send a CD of images, RAW, TIFF, & JPG, all the same shots, so the agent can see what is the problem, but shot on a tripod. There is no reason why the Bigma should not be sharp, even wide open.
At the moment I am also testing a 120-300mm 2.8 Sigma, along with both TC, but more about this lens later, after I have put it through its paces.
With the D300 and D3 and higher ISO usable pix's do we need fast lens. I am having a re-think