I've had a lot of great pics with 80-400 on a D300 body. But for the first few years it frustrated me. It needs really high shutter speeds due to the long focal length and handholding. These days I use 1/800 or faster.
I also find that it is a lot sharper at f7.1 than wide open at f5.6. On my D300 I had to set a small correction in AF fine tuning to get it to focus on target.
On the D300 you can go up to ISO 640 and sometimes ISO 800 without significant deterioration due to noise. Pushing up ISO to these levels gives you the higher shutter speeds.
For small birds I sometimes use a SB800 flash to fill in a bit of light. If using flash do not be tempted to use the Auto-FP high speed sync setting in the belief that 1/320 flash sync speed will be better than 1/250. Auto-FP makes the flash duration longer and the brightness weaker so you are more likely to get blur. Stick to 1/250 max flash sync speed. The flash duration is extremely short so in scenes where flash provides most of the light, the longer shutter speed is less of a problem.