In addition to Cecil's method which appears to work very well, a friend of mine who raised show quality waterfowl and poultry had problems with hawks and owls, he strung overhead lines above the fenced area.
In addition he hung up CD's which would spin in the wind and flash from sunlight during the day and would have some flood lights on at night to reflect and spook the owls. He claimed this especially helped the owl problem at night.
Not sure if this would have an affect on Herons.
