Got to love the internet. Some how it got turned around to using flash????? The local web sites KNOW IT ALL. Responded.. There is definitely a couch with this guys name on it.. LOL
Anyway, I used fill flash for a few years and got pretty proficient at it. But I never did really like it that much. It is a tool that you really have to experiment with. And believe me I have made all the mistakes with flash and birding. IMHO, most people just do not know how to use it properly. And when to just leave it alone. Which by the way is about 99% of the time for me now.
Natural light and learning how to use it is so much more rewarding and makes for a natural stunning pic. Personally getting back to the basics is all that is needed. Buy the lens for the job. These new fast lenses are amazing. And with the new technology in DSLR's ? Just try NOT to use flash for fill. Just try it. I was persuaded by a seasoned low light photographer and it has made all the difference in the world in my low light photography.
Another thing is to shoot to the right. IMO it is crucial. Check the colors on the bird and take the exposure as far as you can without blowing the highlights. It will give you a bright and vibrant photo that will Post process beautifully. I like bright pics. For this you need to shoot in Manual or get proficient with AV and exposure compensation. I shoot in Manual only and have been for a few years now. But many shoot AV and love it. Just what ever you get used to really.
The blog post just under this is a good example of NOT using fill flash. It would of been easy to use and not even think about it. I have heard all the excuses why you need to use fill! IMO they just do not hold up anymore.
Enough for now, go birding... Have some fun while relaxing and watching birdssssss. Nothing like it.