I believe the Magpie Poems are an old, English tradition. That is, seeing two magpies together is an omen of good luck, as the poem so suggests. Seeing one magpie is thought to be bad luck. Hence, one thing you can do should you see a single magpie is to cross your eyes so as to bring about the perception that you are seeing two (changing your bad luck to good luck, perhaps).
Well, that's what I recall in any case. I didn't Google this so I can't offer more reliable sources of information. Nonetheless, enjoy!
One for sorrow,As to the second version, which I will present to you shortly, I vaguely recall a character (Abel I think) in the story saying that it is a secret version of the more well-known version above.
Two for joy,
Three for a girl,
Four for a boy,
Five for silver,
Six for gold,
(And) Seven for a secret
Never to be told.
One for sorrow,And those are the Magpie Poems.
Two for mirth,
Three for a funeral,
Four for a birth,
Five for Heaven,
Six for Hell,
Seven's a devil,
His own sel'.