Hi Kev,
I have the Bigma 150 -500. I use it to shoot my son playing football. (Aussie Rules) It works a treat. I imagine it would be terrific for birding too.
I shoot hand held on the K-7 and turn off the in body stabilisation, figuring that Sigma know how to make in lens stabilisation for their own lenses.
Here is a link to some of the shots. http://www.flickr.com/photos/owizofo...7624243905151/
Cheers
Wayne