I have heard that if you put a fake life-sized heron at your pond, no others
will come because they assume the spot is *taken*.

That's true for a while. You'll find that if you don't move the fake heron
daily, the real herons catch on and ignore it.

Another problem with decoys: I seem to recall reading somewhere (on this ng?)
that someone placed a decoy on the edge of his pond. It was mating season amd a
male heron mistook it for a real female, grabbed all the goldfish and left
their bodies at the feet of the decoy as an offering. (hey, at least he bought
her dinner!)

~ Gary in zone 6a