I don't think you are correct with this Barry. I think these sizes are actually the equivalent already to a 100-400 lens with the sensor size taken into consideration.
Can anyone else clarify this?
No, the Panasonic Leica is actually 100-400mm. The sensor 4/3-type, and in order to get equivalent of 35mm film (the most common reference), the number needs to be multiplied by 2 ("crop factor"). So it's equivalent of 200-800mm on 35mm sensor.
The lens of SX30 is 4.3 - 150.5 mm. The "crop factor" is 5,62, and thus is equivalent of 24-840mm on 35mm system.
A 210mm lens on D70 is equivalent of 315mm on 35mm system (crop factor 1.5).
In practice, a good (sharp, large aperture) 600mm equivalent lens is better than bad 800mm equivalent lens, because the image can be cropped. Cropping magnifies the lens aberrations and softness, as well as digital noise. This is when larger equivalent aperture ("crop factor" x actual F-number) and modern sensors become important. Also when cropping, high megapixel counts become useful.
Nikon 70-300mm lens for Nikon 1 system ("crop factor" 2.8) is small and equivalent to 196-840mm. That lens has a very good reputation and has a manual focus ring. Nikon 1 system is pretty much abandoned, so the prices may be attractive at the moment. Based on what I have read, Nikon 1 + 70-300mm is a very attractive option for birders, mainly because the continuous auto-focus is so good (birds in flight should not be a problem, although it's never easy). It's also a very small system. The image quality should be much much better than your SX30, but when shooting birds, almost nothing is good enough.
There are cheaper alternatives to Panasonic Leica 100-400mm for micro four thirds as well. If you are interested in that system and looking for cheaper alternatives, check out Panasonic 100-300mm (note that mk 2 is much better than mk 1, although optically very very close or even the same), Olympus 70-300mm, Tokina 300 manual focus mirror lens. Olympus 300mm f/4 is also great, but over $2000.
When comparing systems and lenses, it's very useful to multiply both the actual focal lengths and F-numbers with "crop factor" to see the actual differences. The bigger the focal length and smaller the F-number, the better.
