Uuglypher wrote:
Oscar, was the heron actually vocalizing with this posture and behavior?
My guess, if it wasn't, and since you called it "Solent scream" is that it was practicing the thermoregulatory (cooling) technique of "gular flutter" in which the beak is held open and the floor of the oral cavity and pharynx are vibrated or "fluttered" to enhance surface cooling by evaporation of surface moisture.
Just a guess.
Dave
It was completely silent. Thanks for looking