I love real light [sunshine], but if your bird is in the shadow of a tree, you can use the fill flash to help you a little.
That's my thinking and preference. The area I was photographing the robin is a heavily shaded, tall treed environment. In a few months, the bushes close to the ground (prickly current bush) will be laden with berries and these little birds will be in that light, which is more even and better balanced. So, in this case, I'll just go with natural light.
As the robins were around 3-4m above me, the back-lit sky was causing havoc with the metering. As it was, I had the camera over expose by 2/3 stop, and the fill flash run 1/3 stop under, so it didn't "look" like a flash image. Well, that was my logic of the camera set up.