Hi Simon, i own the sigma 120-400 lens i use for taking birds its a heavy lens, which i nearly always use on a tripod. I got some good sharp images from it too. I wish though i had bought the 150 - 500 sigma maybe a couple of hundred quid more but worth it for the extra reach.
Check out these images of brians on this link http://www.ephotozine.com/user/brian65-101882 he uses the sigma 120- 400 and has took almost all his bird pictures with it hope this helps
Check out these images of brians on this link http://www.ephotozine.com/user/brian65-101882 he uses the sigma 120- 400 and has took almost all his bird pictures with it hope this helps