Thanks for the plug
but the birdforum galleries are more of a visual recommendation than my site.
Birdforum galleries to look at to evaluate the Sigma 500/4.5 credentials
http://www.birdforum.net/gallery/sho...00&ppuser=3294
and
http://www.birdforum.net/gallery/sho.../3/ppuser/5522
and
http://www.birdforum.net/gallery/sho...00&ppuser=5678
and
http://www.birdforum.net/gallery/sho...0&ppuser=25708
There's quite a few others as well... which goes to prove that this lens is a fabulous lens for the bird photographer, without the considerable cost and weight of the 500/4 and 600/4 prime lenses.
A used version for under £1500 (make sure it is EX HSM version or newer) is pretty much irresistable.
cheers,
Andy