Why do egrets and herons fly with their neck retracted in a tight S-curve,but equally long-necked and/or long-legged birds like cranes, swans, and storks fly with their neck stretched straight out? As one blog reader suggested in the previous post, it’s all about center of mass. A bird’s center of gravity (or the center of mass affected by gravity — i.e., the balance point at which all external forces are concentrated) is located at or very near the shoulder joint where the wing articulates with the axial skeleton.
A large-bodied bird like a swan or goose or crane stretches its neck out when it flies to offset the amount of body mass it carries to the rear of its shoulder. Conversely, herons and egrets carry much less body mass behind the wing and are relative light-weights for such tall, long-legged birds. Therefore, they bring their heads back allowing the mass of the long legs to offset elongated head and bill. I hope the illustration below demonstrates this point better than I can in words.
Even though retracting the neck inward might increase the amount of drag during flight, due to the larger surface area meeting the air mass, herons and egrets have maximized the lift the wings provide with their low body weight and large wing area. Some data to illustrate this point:
The implications of these differences in body mass, wing area, and wing load are that Great Blue Herons can fly relatively slowly over the marsh while they search, with minimal cost for take-off, while Trumpeter Swans and Cranes must power up and fly with greater speed to get equivalent lift from their wings.
The same sorts of trade-offs (wing loading vs lift and speed) are seen in modern aircraft, which are compared with a variety of animal flyers in the figure below.