As a young lad I was told to say 'Good Morning, Mrs Magpie' - but I can't remember if that was from my parents [Scottish] or locals in the Cotswolds, where I lived. Apparently, as magpies mate for life [this may not be true] a single magpie must have lost its partner so to ignore it would be bad luck. I still say 'Good morning/afternoon/evening' to lone magpies if I remember.