Thanks for the comments everyone - much appreciated.
Arthur, there is a very small halo on the top of the head - it's not on the RAW - so its obviously a sharpen-for-web halo. I find particular colour/tone combinations problematic with halos - especially bright reds against bright greens - but this one is easily fixed, so thanks very much for making the time to comment and pointing it out :-)
Also, this is a setup, with the birds attracted on to the perch which is next to a seed bowl. It's interesting to note that this setup didn't last long at all, as several rosellas took a shine to the flowers, and proceeded to rip it apart!
Cheers, Greg