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1.
The belief by pirates in the 1500's and 1600's that black magpies native to the Caribbean would detect and seek out the fermenting alcohol of naturally occurring molasses from sugar cane plants which produced natural rum. During specific times of the year, P. hudsonia would roost in the Caribbean during migration. The migration traverse would lead the birds to search high energy food. Of particular interest to pirates was rum. The pirates would watch the seasonal migration patterns of the birds as well as roosting locations in hopes of finding free alcoholic booty to trade on the high seas as well as use for personal inebriation. The alcohol was easily captured and stored for ocean voyages.

Currently being used as slang for people too cheap to buy their own liquor when going to parties who steal from others.
The Jolly Roger is circling the east coast of that island because of the Black Magpie Theory.

or

Keep an eye on your rum in the freezer. That random dude is thinking Black Magpie Theory.
by grs_bmt December 04, 2010