Magpies normaly have one partner (as do many birds) for life. If you see one one its own its possible that its 'other half' has died.
As far as I am aware that is why is is unlucky to see see one magpe,.... that is from the magpies point of view.
One superstition is that the magpie is the Devil's disguise on earth - if you see one, you should salute and say 'Good morning Captain.' to divert the Devil's spell.
My friend drives us mad with 'saluting' magpies - she goes on about 'good morning mr magpie hows your wife and kids' every time she sees one and does a US army style salute. I have never done it and nothing has ever happened to m..aaaargh