Great Egrets are very territorial and Great Blue Herons tend to leave their nests unguarded. Not a good combination. Herons do seem to wait until the chicks are a few weeks old. However compared to an adult egret the chicks are very small.
Egrets will steal branches from another nest. They will also take over a nest, even if it is occupied. A large undefended nest is fair game.
When an adult Egret finds a nest occupied they have a very structured method of testing the occupants through intimidation.
The breeding plumage fanned out and a up and down motion, repeated for what me be an hour, will make the Egret look very large and intimidating.
Also over the time they are displaying their size a bird will edge closer to the nest. This allows them to determine how safe it is for them and how dangerous the occupants may be.
At any time this is happening the adults can return. The Egret is aware and will leave if an adult approaches. If not they will continue with the intimidation and move to the nest.
Depending on the situation sticks and branches can be repeatedly stolen. An attack on a nest can also occur also.