You have several technical challenges in this situation. Even if it's an image stabilized lens, 1/100 just won't be fast enough for hand holding at 400mm. You might be okay with a monopod and IS. With a tripod it wouldn't be an issue. If you have to shoot with a slower shutter than you would like, take lots of exposures: it increases the chances of getting a few sharp ones.
Try increasing the ISO. My Rebel XT was good to 800, my 40D is very good to 1600. I don't know about the XTi.
A flash would help, but it raises some other potential issues like disturbing the bird, etc.
In extreme contrast like this with a very dark subject and very bright background, you should probably try +2 exposure compensation. Even then, those sharp black/white transitions will bring out the worst in any lens.
What photo software are you using? Photoshop has several tools for helping reduce highlights and lighten dark areas, but any other software should have some tools to help. Shooting in RAW format will help if your software supports it.
BB