Youtube aficionados may have already seen this video. Once the giggles die down, consider the mechanism that makes this possible! (The chicken makes a loud noise at the end, which thoroughly frightened my coworkers. Just a warning if you're not wearing headphones.)

Hummingbirds are also good at keeping their heads very still while they flap their wings furiously:

The purpose of this, for the chicken, is to keep the her wits about her - i.e. make sure she knows which way is up, and where potential threats are in relation to her. To do it, the chicken has to have sensory information flooding into her brain, telling her how her body is moving.

The technique - which you could generally call "tracking" but is also pretty much the same thing as "dead reckoning" (or is it ded reckoning?) - is utilized by aircraft and some car navigation systems. (I love it when "high tech" turns up in Nature.) The chicken's body communicates its movements so well with her head, that she can almost instantaneously compensate for her movement of the lower body, and keep her head stationary in relation to her environment. To do this, her body has to have some fixed point, some center, and determine how far her bum has moved away it, then move her head an equal but opposite distance from it. Once again this requires very rapid communication, and then action, on the part of her body.

This is also an interesting lesson in relativity. Her head is definitely not moving in relation to the background; we can clearly see that it remains at the same coordinates. But in relation to her own body, her head is moving. When her body is dropped down low, her neck stretches out, moving her head further away from her heart. When her body is lifted up, her neck shrinks down, moving her head closer to her heart. Her head moves an equal distance, but in the opposite direction, to the rest of her. She is compensating for the motion of her body, in order to keep the orientation of her head and her environment the same.

You also see bull riders trying to do this - keep your head steady while your lower body moves. (Not that I'm advocating this sport or the music in this video):

There are devices that can do this for moving vehicles, for use when GPS systems can't get a signal from the parent satellite. These Inertial Measurement Units utilize dead reckoning to determine the vehicle's location based on a fixed point. Based on the last location point gathered from the satellite, the car can keep track of how far you've gone (how fast and how long), and how you've turned the steering wheel, to figure out where you should be.

While such a device might seem pretty unsophisticated in a car navigation system ("It figured out that we took two right turns? Big deal.), it can be much more important in aircraft. With the open sky as your highway, it would be easy think you were heading in a straight line, but find out too late that you were one degree off and were now many miles from your target. Kind of like when you're putting up a shelf and decide not to use a level, then realize it's lopsided a little too late.

That's not to say that IMUs are perfect, or better than GPS systems. In fact, even very advanced ones suffer from the fact that minor miscalculations can end up in very large mistakes (so your plane still might end up hundreds of miles from your destination). Their biggest advantage is that they can be analog, or don't rely on satellites service to operate. And chickens hate it when they can't get service.

1. Where do mechanical or spring-balanced systems like a steady-cam (or other open loop systems) fit into the picture?

2. Dan,

Open loop systems rely on damping. In effect, they filter out the bumps rather than counteracting them. Shock absorbers on most cars work that way, although at least one company (Bose) has developed some pretty impressive active feedback damping systems. You can see a side by side comparison of active and passive suspensions here http://www.youtube.com/watch?v=-3Wvibgvyt4

-Buzz

3. Here is an important question what sensory data does a chicken use? Would she still be able to keep balanced if her head was covered so she wouldn't be able to see?

4. This is because the eyes are connected to your nervous system and the possibility of something in the nervous system affecting your eyes is high blurry vision in one eye

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