Here is a link to a serious look at Sigma 300mm f2.8 prime and 500mm f4.5 prime for birding, & some truely amazing & inspirational Bird Photography.

http://forums.dpreview.com/forums/re...ssage=39125917

http://www.flickr.com/photos/avianphotos

http://www.pbase.com/barryb/sigma500

Cheers
Col