Really good start and what an exciting subject! Fast shutter speeds are critical when you're both hand holding a shooting a quickly moving bird. I wouldn't hesitate to crank up the ISO a bit more. With my D200, I rarely let it drop below ISO 600 for birds and wouldn't hesitate to go to 800 if I needed more shutter speed. The D90 should handle higher ISO better than my D200 did. Even with a tripod and gimbal head, it can be difficult to track flying birds, so handheld with a 300mm was a good way to go. High speed continuous shutter release is also important - you'll get a lot of throw aways, but a couple of good keepers make it all worthwhile.
Warning, if birds are going to be your pursuit, you will spend a LOT of money on lenses and gear to support them.