Howdy -
I've observed Great Blue Herons defending what I assume to be feeding
territories, outside the beeding season. Usually the looser will vacate the
area before damage is done. I would think the simplest explanation for this
case would be territorial behavior gone to the extreme.

** In one case of territoriality, I observed a heron chasing another out of
its area and force it to the water away from the shoreline.

--
Charles Swift
Moscow, ID
chaetura at gmail.com