Looking for Trouble

Magpies are, for the first time, nesting in our garden but I think this one is on the prowl looking for Blackbird nests to raid. The Blackbirds have been nesting in our garden for years, along with the Wood Pigeons and sundry small birds. The Magpies are always a pest but I wouldn't disturb their nesting but don't like them raiding other birds nests.
Not a lot you can do to stop them really, as soon as your back is turned they come back to carry on. Beautiful birds, but not nice.