What to do with your neck when you fly…

Why do egrets and herons fly with their neck retracted in a tight S-curve,

Great Blue Heron image by Alan D. Wilson, http://www.naturespicsonline.com from Wikimedia Commons

but equally long-necked and/or long-legged birds like cranes, swans, and storks fly with their neck stretched straight out?

Whooping Crane image by John Noll from Wikimedia Commons

Trumpeter Swan image by WildReturn  from Wikimedia Commons

Saddle-billed Stork (the tallest stork) in flight. Image by Sea1109  from Wikimedia Commons.

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.

Image of long-necked flyers are scaled to the same length (black line) and center of mass is indicated by the red line. Rear-heavy birds (crane, swan, and stork) balance by extending their necks in flight. Small-bodied herons and egret pull the neck inward to balance.

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:

Wing loading (weight per wing area) of Great Blue Herons is much less than for Trumpeter Swans and Whooping Cranes.

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.

This figure is from an excellent website on Bird Flight.