I came across this perfectly posed mistletoe bird in a patch of dense wattle scrub. I was walking with the Sigma 50-500 lens, with settings for the bright sunlight that we had been in moments before. I took several shots on the 'custom' position that I had set up for birding but this was giving me underexposed images. My flash unit was back in the car, and it was a bit much to hope that he'd wait long in this position. So I just flicked the dial to full automatic, allowing the on-camera flash to pop up. This was slow-speed flash and there was sufficient ambient light to give a blurred outline to the moving bird.
He jumped noticably when the flash went off, and stopped singing to concentrate on me; birds are affected by flash light. However flash when used as a fill to reduce shadows and add a catch light produces wonderful bird images.
Secrets of Digital Bird Photography offers a range of tips on the effective use of flash in bird photography.