The manner in which Canon adjusts the sensor signal for various ISO settings doesn't follow what might be expected – progressive increases in noise as the ISO is increased – Canon does some fiddling mathematically to 'create' some of the ISO settings, as a result at some ISO settings the noise goes up when you expect it to go down. Following is a link that discusses this specifically for the 30D, these data help establish expectations for noise and ISO.
http://forums.dpreview.com/forums/read.asp?forum=1019&message=19721647You can try different settings and see what works best.
Some general information the types of noise:
http://theory.uchicago.edu/~ejm/pix/20d/tests/noise/index.htmlOn the 30D I shoot at ISO800, and occasionally higher only if I have no choice. Noise is predominant in the shadows and dark areas, where the signal is low relative to the noise (low S/N), and as we pull up the shadow detail in post processing the noise follows. I shoot RAW and "expose to the right" while minimizing the overexposed areas as much as possible in order to reduce shadows/dark areas. Inevitably I have to do some noise reduction in post processing.
There are commercial products available to do this as Dave mentioned, and DPP can also be used as Brad pointed out. The chrominance noise setting reduces color noise and can be somewhat effective. I try to be careful in setting the luminance NR as this has a big impact on desired details in the photo, but usually I do the NR in Photoshop.
I use a Photoshop action, ISO PS NR, written by Dave Marsit that has 4 selectable levels of NR. I found this one to work well for reducing small amounts of noise without compromising the photo details. The action is provided for free, has been around for several years, I've used it on PS7 through CS3, it’s worth a try. Here's a link with some examples of the performance, unfortunately the download link is dead.
http://www.canonians.com/noise.htmYou should be able to download it from here:
http://www.geocities.com/rnlnero/PE2stuff2.htmlThere are other photographer-contributed PS actions you could try as well; I suggest a Google search on free Photoshop actions.
Lastly is the lens, I have the same one and do not use it for birds. Mary made a good recommendation with the 400/5.6, 300mm is a little short for birding, you have to crop a lot unless the birds are quite close, also the 70-300 DO is not a very sharp lens.
Good luck.