Early summer 2010, I made the decision to buy an external flash (SB800) to better my bird photography. At first, I was hesitant about using it because I didn't know too much about how to use it. After a few quick tips from a friend I was off to the races with it.
Nikon D90, Sigma 150-500 OS, SB800 set to TTL Balanced Light +3ev w/ Better Beamer
The Purple Martin photo about was from one of my first outings with the flash. I was instantly amazed with the difference in color and detail I was able to pull out of the feathers. The Purple Martin photo is a good example of how effective fill flash can be.
Nikon D90, Nikkor 300mm f2.8 w/ TC14 @420mm, SB800 set at TTL Balanced Light +2ev
Here is another example of fill flash on bird photography that I feel really helped the image. In this situation I actually had pretty good light (which we all know doesn't happen often). The flash helped fill in the shadow areas on the perch and bird to make the light more even across the photo.
Now, the SB800 and couple sets of batteries are always in the bag ready to go.