Magpie "anting"

I was interested to see a Magpie "anting" itself under it's wings at Shibden today. It's a long time since I saw this, and I think I've only seen Blackbird do it. When the bird had flown off, I went over and it had been stood over a nest of small red ants which were swarming out in a break in the the relentless rain. Birds use the ants as an insecticide to get rid of their skin parasites, I seem to memory, but I can't find anything about it in any of my books.