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My plastic bird bath stays perfectly still in up to 70MPH gusts, and it's not even tethered down!
It has almost the same shape as that bird feeder pictured, but relieved on top to hold water (of course).
I think that the cross-section plays into the equation immensely. It seems like my bird bath acted as an inverted wing and created down force as the wind blew across it. The water adding weight was helping too.
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