I experienced the same thing recently though not caught in brambles like yours, I found a young magpie on my decking, sat looking ever so sorry for itself, again I was like you thinking what shall I do, hubby said it's parents will come back to feed, but I was not so sure. I never actually witnessed it being fed but saw the parents either in the trees overlooking the garden or on the fence, and they were never far away. The next morning I found it sitting on a little branch overhanging the pond ever so close to the water and I was worried that it might drown and I walked up to it and it went onto the decking. It was in my garden for a week and each day was able to sit higher in the branches in the bush which it had made it's home. Eventually, it grew it tail feathers and was able to join it's parents. It came back a couple of times and I was able to take a photo of it which I have put on the
Gallery. Hopefully your magpie will have the same happy ending as mine. Please let us know how it gets on.
Barbara