The engineer in me is always intrigued – birds need to keep their center of gravity just in front of their center of lift (wings) for stability.  In big, meaty waterbirds that float and paddle (geese, ducks), they retract the feet and have to fly with necks straight out ahead to achieve balance.  Big wading birds with smaller body proportions, like this Great Blue Heron, have to pull their heads back to avoid being front-heavy when they fly, and keep their feet stretched out behind to avoid making it worse.  Their necks stay S-shaped in flight, even though their natural resting position on the ground is with necks upright.

Photo Credit – Lynn Abbott