Turned out there were huge numbers of ladybirds wintering in the corn stalks. Next summer I'm going to plant some corn and other vege and flower plants in a corner of the garden and leave it "wild" specifically to encourage and shelter beneficial insects.
My other chickens have been getting very naughty about flying over the fence and into my garden, so have had to put netting over their run and start keeping them contained. My chicken flock numbers 40 now - some headed for the freezer in the near future.