David Smith said:
Now, I am new to digital but when using film I don't think I ever used more than 400 ISO. Is high ISO used much ?
The higher the better. The longer your lens is, the more important it becomes to keep your shutter speed up to reduce subject movement and camera shake. IS is no panacea - you have to get the shutter speed up as high as you can even with the best IS in the world. (The experts say that lens-based IS is superior to camera-based IS, BTW, particularly so with longer lenses.)
400 ISO is the starting point for most people doing bird work, with 800 there when required. With a good sensor (the 8MP Canon one is the best within any sensible price range) you can go as high as 3200 if you have to, and still wind up with a quality image. See
http://www.birdforum.net/pp_gallery/showphoto.php/photo/134943 for an example at 3200. Now that was taken at 1/90th of a second (
way too slow for a 500mm lens, even using a good tripod, but the light was truly dreadful), consider where you would be with a noisy camera like the Sony that restricts you to 400 ISO max ..... You'd be at 1/11th of a second ... well, actually, you'd be worse, as you can't get an f/4 lens for the Sony, and at f/5.6 you are down to a 5th of a second.
OK, that was an extreme example, and a decent shot that resulted
despite not having any light to work with. A good more general rule of thumb for bird photography on digital SLRs is that you want your shutter speed to be twice as high as your focal length. (The old 200mm = a 200th of a second "rule" is more appropriate to film and full-frame sensors, which we are not considering here.) With a 400mm lens, that translates to needing around 1/800th. This is where your ISO comes in. Unless you can afford an f/2.8 lens (around $US10,000 for a 400mm one) and someone strong to carry it for you, you need to push the ISO to get decent results. That, essentially, means you should be selecting between Canon 30D, Canon 20D, Nikon D70, Canon 350D, Canon 400D - pretty much in that order. Note that I put the 10MP 400D last after the 8MP Canons and the 6MP Nikon: this is no accident. I own a 400D and the extra 2MP delivers no worthwhile extra detail over my 20Ds, but the extra noise at anything over 400 ISO is noticable.
Keith (who posted above) owned a Nikon D200 for a while (which uses the same sensor as the Sony and the Pentax K10D) and quickly sold it. Nice camera, he said, but no good for bird work - too noisy.
Finally, the really important part: lenses. Lenses are much more important than cameras, and this is where Canon and Nikon have it all over Sony and Pentax. If you want to shoot birds, you pretty much have to go Canon, or else Nikon. The smaller brands have (by all reports) some excellent modern shorter glass, but nothing to compete with Canon and Nikon in the 400mm class.
There is absolutely no way that you will get decent results from a super-zoom. No lens maker on the planet has yet figured out how to make a lens zoom from 17mm to 200mm and still deliver a quality image. It's probably not possible. You need a good deal more than 200mm for birds in any case: take 400mm as your starting point. If you can't afford the excellent lenses in this class offered by Canon and Nikon, there are some very usable third-party products. The Sigma 50-500 is heavy and a bit slow but well-regarded, and there are several others.
Chose well and good luck!