I've been watching the behavior of a small colony of Great Blue Herons. Three - maybe four nests in the stop of a pine tree.
I've found a very useful reference: Pair-formation displays in Great Blue Herons.
I've noticed that pairs of Great Blue Herons make a slapping sound that seems associated with "kissing" - grabbing each other's bill and then drawing back which creates the slapping sound. Apparently this display is called bill slapping (or something similar). here are some examples.
One grabs the bill of the other
There is an alternating thrust of one and then the other
Here the left heron retreats
and the right heron starts a thrust
Right heron thrusting
Wing flapping to stabilize the right heron's position
Finally he is tired of this display and leaves