Ah, yes of course. Having a tripod is fine for getting over the poor light problem but only if you're subject is still but as you say you're tied with the max. f5.6. I can't think of a bird that remains still. This is why bird photographers go for f4 and f2.8 lenses but look at the prices. The Canon 70-200mm f4 IS is £714 , the f2.8 version is £1173 (on offer at Jessops atm). The 400mm prime f2.8 IS is £4,794 . The 600mm (a favourite with birders) at f4 IS is £5279. There's even an 800mm IS but it's f5.6 and that's £7724. I was aware that my 100-400mm (f5.6 at 300-400mm) was not going to be good in poor light unless I could tripod it and it would have to be a static subject. Photograophy is an expensive hobby isn't it if you're photographinh in poor light and a moving subject. Have you tried the Noise - Ninja ? was thinking of getting it myself when the christams rush is over. Reports of it's efficiency are very good.
I like this site. See the bottom of the review for. plug in for PS.
http://www.luminous-landscape.com/reviews/software/noise-ninja.shtmlhttp://www.picturecode.com/ 
