The photo attached here is of a seldom seen event. When an unattached GBH male is looking to attract a mate he will adopt this very stretched pose reaching to get every inch of height he can achieve while making a "goooo goo" sound. This is followed by several sharp snaps of his beak. Apparently this long lean look is VERY exciting to the ladies. Great Blues will also adopt this stretched look when flying during the mating season. During non-mating periods they generally curve their necks in an "s" shape while in flight, but during mating they really try to elongate their appearance.
D300s with 200-400mm lens, tripod, Sidekick, fill flash ISO 400, F5.6 1/249