Why Don’t Birds Break Their Necks On Deepwater Dives?

(Inside Science Currents Blog) -- Animals perform many feats that are remarkable once you think about them. Here’s one that I never previously contemplated: seabirds dive into the water to capture fish at seemingly breakneck speeds — yet their necks are completely unharmed.

Speaking at a meeting of the American Physical Society today in San Antonio, Sunny Jung of Virginia Tech in Blacksburg and his colleagues sought to understand how birds survive these dives, by studying a type of seabird known as the northern gannet. Up to three and a half feet long, these elegant birds look like torpedoes when they fold in their wings.

The birds experience tremendous impact forces as they smash into the water, which is a thousand times denser than air. According to Jung, northern gannets hit the water at speeds of approximately 55 miles per hour. Imagine driving your car into a wall of water at those speeds, he said.

Gannets perform about 60 dives per hunting trip. They enter the water at speeds comparable to those of the fish moving in water. Birds’ necks are remarkable in comparison with humans. Humans have 7 neck bones while birds have anywhere from 11-25 bones. The neck can be as much as half the length of the body.

Here's what happens when the bird hits the water - just remember, the bird's neck remains stable and doesn't buckle during this process.

(Video courtesy Sunny Jung, Virginia Tech, et al.)

As birds enter the water, there is a large compressive force on the neck, with downward gravity pushing against the neck in one direction, and the upward impact force pushing in the other direction. As the birds enter the water, gravity still pulls on the birds but the upward force is now a drag force caused by the water. An air cavity forms around the bird's neck. 

In a collaboration with the Smithsonian Institution's National Museum of Natural History, Jung and his colleagues studied this process in various ways. First, they took dead birds that were frozen and dropped them in the water. They also took CT scans of the birds and created 3-D printed replicas of their bodies, and dropped those replicas into the water. These laboratory tests replicated the birds’ natural dives pretty well. They made videos that provided insights into the physics of the impacts.

The researchers also constructed a model of the birds, using a cone for the beak and head, connected by a string-like elastic beam for the neck, and a small cylindrical weight on the other end for the rest of the body. Dropping these model birds into the water, they varied their impact speed, and the angle of impact with the water. They also used models with different neck lengths.

The researchers found that there were two outcomes when the model birds dropped into the water: either their necks stayed straight, or they buckled up. Jung explained what makes the difference is for the birds to dive slowly enough to dive safely. If they dive faster than a certain speed, their necks would buckle up, according to their experimental models.

In addition, longer necks in their model birds would buckle more easily, something that is always seen when dropping any string-and-weight model to the ground. Gannets tend to have longer necks compared to other birds, bringing them closer to the danger zone for higher impact speeds. But long necks confer other advantages for birds, such as being able to dip deeper into water for food.

The researchers are going to study the birds’ diving dynamics further. But they are already starting to contemplate another subject: how humans are able to dive safely into the water.

Ben P. Stein, director of Inside Science, has covered physics as a science writer and editor since 1991; he tweets at @bensteinscience.