Around this time of year, Common Mergansers cross the U.S.-Canadian border on their way to wintering grounds in the Lower 48. But how do they know when to go? Ducks are well insulated against frigid winter temperatures, but mergansers can find their fishy prey only by diving below the surface of open water. So they’re doing just fine, resting and feeding in southern Canada, until a thin veneer of ice forms on their lake, signaling the time has arrived to head south!

More info and transcript at BirdNote.org. 

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