jamie: I primarily use the flash on bird shots because you will find that the oil in the plumage can cause a shine and you will get more vibrant colour.
Also a little highlight in the eye (which I like also)
If you have a smart flash (like the sb800 for instance) you can tell the flash how much to use. Typically 1 EV less than you would use if the flash was providing the main light. This way you wont have the flash blow the subject away with too much light but will provide perhaps some highlights or light some shadow area a little.
I only found out that these things have whiskers today also!!
As for looking fake, I made another post today:
http://www.d70users.com/viewtopic.php?t=9615
which was the same wren but without the sky as a background and I thought it looked more natural, but didnt have the sun on the throat plumage. At this time of year though these things have breeding plumage and look even brighter than usual, so the colour on the throat is actually the way they are right now
The second shot in this post has the wren's rump casting a shadow on its head and making it too dark for my taste.
Posted it cos I thought it was cute!!
Steve