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Artificial Refugium rule
(slrpnk.net)
Be sure to follow the rule before you head out.
Rule: You must post before you leave.
Some species of bats spend more time in the upper ranges that no human can hear than others.
The spotted bat for instance, is found on west coast if North America and mostly calls at 11khz well within even older human hearing while other bats operate entirely outside human hearing.