Hi mark.
I use the Sigma 100 - 300 f4 APO IF HSM model and there own 1.4 EX TC. I bought the lens on, among many, this review 's'.
http://www.fredmiranda.com/reviews/showproduct.php?product=103&sort=7&cat=37&page=1.
It is a very fast and silent internally focusing lens. Extremely sharp, contrasty and colour balanced and gives a nice bokeh too boot. On the downside it is heavy and not as long as I would like for birding. The 50 - 500 is a better option for general birding, quite a few of the photographers I see around hides use this lens, but the trumpet zoom is not to everybody's liking and it is not as good optically as the 100 - 300.
As a comparison my friend uses the Canon 100 - 400 L series lens and he has used my Sigma on his body, he reckons is optically as good and snaps into focus quicker. No data back up to support this, just hands and image on conclusions.
At the end of the day it is short for a birding lens, but all lenses are to short if you can't get close enough !!!! I'd recommend it as a long zoom, it has given my some great images and fault free service. I've attached an example of the sort of capture. This Swan was about 15 foot away, no TC was fitted at this time but also no post image cropping, i.e this was the view from the lens. Not the most imaginative shot but gives you a base to work from. If you want to see other images let me know.............................
Hope this helps
Jason