Almost all sites that treat the Great Blue Heron give a passing reference to the plumes that extend from the back of the head. When mentioning them, one site explicitly says they are decorative.
Here is a picture of the Great Blue Heron that was taken in November and probably hatched that year. Consequently the dome of the head had not gone completely white and the upper mandible is blue. Nevertheless, the feather plumes have already grown down the back of the head.
This is yesterday’s heron. The adult’s white cap is visible and the upper mandible has become the breeding colour of yellow. The feather plumes extend further down the back of the neck. The question is: Are they functional or decorative? Certainly they look decorative, but then the bird is not flying.
This bird then flew off and the function of the plumes became evident. They provided a smooth flow of air from the head across the back. Recall, that the Great Blue Heron flies with its neck tucked against its back. Not every large bird has such a flexible neck. The crane must fly with its neck extended. Now, look at the plumes coming off the back of the head. The air flows smoothly from the head and along the back making it one continuous surface and thus minimizes drag. The plumes are not decorative, they are functional.