20-29 mallards is a lot on just 1/3 acre. They are adding a lot of "fertility" to the water, more than most of us would care for. I have no more than 3 mallards hanging around my ponds regularly (Huey, Dewey, and Louise) and am willing to put up with their contributions to water fertility. But 20, I would draw the line with.

"Live like you'll die tomorrow, but manage your grass like you'll live forever."
-S. M. Stirling
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