Oh, great - a recommendation for a good curated European bird photography site! I haven't been to Europe, I know nothing about the bird species, this must be a good site to learn from.
I bought my birding set-up 3 years ago - EF 400mm f/5.6L no-IS, used on my 60D. I also had (and have) a 70-200 f/4 L IS and a 1.4x TC II, so there's some broad focal length coverage. I might have gone for the 100-400 if I didn't have the 70-200 already. The 400 was therefore strictly aimed at bird photography, where maximum possible focal length is ideal. I wanted to do bird-in-flight photography, and the 400 f/5.6 was highly recommended for that due to fast AF and feather-light weight of 1.25 kg. I was willing to accept the steep learning curve of shooting without IS. I had thought that IS would be completely useless for fast shutter speeds, but it turns out that even shooting at 1/2000 and higher requires excellent (learnable with a little practice) panning skills to get a tack-sharp BIF. I don't know if IS would have helped me get good shots earlier on. For stationary bird shots, I developed some better lens hand holding technique, and started using a monopod for "stake-out" shots.
If I were to start now, particularly without a pre-existing 70-200 f/4L IS, I would definitely be looking at the Tamron 150-600 (1.95 kg), and maybe the upcoming Sigma Sport (2.86 kg) or Contemporary (less than 2.8 kg, more than 1.9 kg, I presume - weight not shown at B and H page). However, I love my featherweight "toy lens" - it is so well balanced, it is a pleasure to use, and I can hike all day without it feeling heavy. I have put in the sweat learning this no-IS lens, so I feel attached to it. Next purchase will likely be the 7D2 body. My attitude about ISO is, push it as high as you need to, and don't worry about noise. Better noisy than motion blurred.