sweetcorn pests

Been blaming mice and pigeons for nibbling my sweetcorn over the last few days but I went to my allotment site on sunday afternoon and I noticed that I was the only one there. As I approached my plot I saw a magpie on top of one of my sweetcorn plants. I went to scare it off but suddenly seven other magpies flew out of my sweetcorn they had a real feast. Never seen that many magpies together before, I've never ever seen one on sweetcorn. I have netted the ones that are still there before they all get ruined.