Hi Lesley, it's a difficult one to answer as at first I thought either the magpie had been bathing in something like an ochre dyke (especially if you live near Sheffield ! ) or it had been injured and it was blood that had spread on the plumage as I've seen some vicious attacks amongst magpies, however I believe there is an abnormality in birds called erythrism which can show up as red/pinkish although this is very rare. It has been seen in magpies before. So, to be honest, I can't give a categorical answer but hopefully somebody more knowledgeable on this forum can give their opinions. My guess would be Erythrism.
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Regards, Hazel