How did the chicken cross the road?

March 7th, 2023

Written by: Barnes Jannuzi

“Why did the Chicken cross the road?” This old and bad anti-joke is a great example of how we often get caught up in the meaning of things, the “whys?”. It is time to take a step back to realize the amazing and beautiful “hows?” of life. Today instead of asking why the chicken crossed the road, let’s attempt to better understand how it did it in the first place.

Have you ever seen a chicken walk? It is a little goofy looking. The chicken appears to bob its head back and forth, forward and backward, in often very abrupt and jerking movements. In fact, their heads never move backward, but they are thrust forward, then stop in place while the body moves forward, before their heads are thrust forward again¹. This motion is not just a silly product of evolution, but an amazing behavioral mechanism to hold the bird’s head very still in place for the most amount of time during its walk. This phenomenon is not limited to chickens, nor to walking². This amazing video of a red-tailed hawk shows the bird’s head fixed in place mid flight! If your first thought was “wow I wonder what prey that bird is looking at” you are not alone, and this lends great insight into why the bird is doing this. It is looking at something, and holding its head in place helps the bird to see better.

How does a fixed head help improve vision? The way that eyes and brains turn light into vision requires some time and consistency in what we are looking at. (Don’t believe me? Try shaking your head as fast as you can without looking at something in particular and see how blurry and out of focus your world becomes.) By holding their heads almost perfectly in place³, the chicken and the hawk improve their ability to see dramatically. Humans also have a mechanism to provide visual stability, but it is with eye movements. As you are reading this, you can slowly rotate your head any way you want, as long as your eyes are able to see the screen, you are able to read the words without a problem. However, many birds can barely move their eyes in a different direction than their heads, if at all. For example, raptor birds like the hawk literally have bones around their eyes, locking them in place, making head stabilization all the more important. This means that the hawk would have a difficult time reading the rest of this post if they were unable to move their heads to look at each word.

Holding your head in place is a difficult task and requires very precise coordination between the sensory information your brain receives and the muscle movements in your neck. If you want to see for yourself, go look in a mirror and try to move your body forward/backward/left/right/up/down and see if you can keep your head as still as that hawk. So how does the bird’s brain accomplish this impressive feat? Through combining sensory input from vision, body position, and balance the bird can monitor if their head is moving in space and quickly make neck adjustments to keep their head still. Let’s take a closer look at each of these senses and how they help the bird stay focused.

Visual sensory input. Head movement changes the way light coming from the environment hits the eyes. For example: when the head moves, far away objects appear to move less than close up objects (You can notice this when looking out the side of your car on the highway, the horizon appears to move slower than the road next to you.) Brains can compute these slight differences in visual sensory input and create an estimate of how much the head has moved. This estimate can then be used to guide movements in the opposite direction to hold the head in place.

Proprioceptive sensory input. Proprioception is a sense similar to vision or hearing, and senses the body’s position in space. Instead of eyes or ears, this sense uses tiny sensory organs in muscles and tendons that detect muscle position by measuring the stretch, movement, and position of muscles and tendons. The brain receives input from these sensors and uses it to determine an up to date estimate of where each limb of the body is in relation to each other, as well as a predictive estimate of where it will be in the near future. Using these estimates, the brain can compute how the body is moving and make additional movements to hold the head in place now and during future movements. (for more information on motion planning and sensory expectation, check out this article.)

Vestibular sensory input. The vestibular system helps to determine changes in head rotation and acceleration. The semicircular canals of the inner ear are a bizarre looking sensory structure that is crucial in detecting head rotations (see figure 1). Each of the yellow crescents are filled with fluid and are lined with sensors to detect motion in that fluid. When the head moves, the canals move with it, but the fluid gets “left behind” and “moves” relative to the canals. The canals can sense the displacement of the fluid and relay that information to the brain. The brain can use this displacement of the fluid from each of these canals to determine how much the head has rotated and in what direction. With this information, the brain can start body movements to compensate. (Fun fact: this is why you get dizzy after spinning in a circle too many times. Spinning causes the fluid in these canals to be so disrupted that your brain does not know how to make sense of the information and mistakenly thinks your head is rotating when it is not!)

By combining all of these sensory inputs and expectations, the bird’s brain generates an expectation of how much the head has moved recently, and will move soon. It then produces neck and body movements to compensate, allowing the head to remain fixed in place.  Just how much of each sensory input is important for these complex movements is still an area of active debate, and seems to depend not only on the particular animal, but also the exact type of rotation and movement being accounted for⁴. However, I hope we can agree that at least in this case, “How” the chicken crossed the road is significantly more interesting than “Why”… which in case you were wondering… was to get to the other side.

References

1. Dunlap, Knight, and O H Mowrer. “Head Movements and Eye Functions of Birds.” Journal of comparative psychology. 11.1 (1930): 99–113. Web.

2. Iwaniuk, Andrew N, and Douglas RW Wylie. “Neural Specialization for Hovering in Hummingbirds: Hypertrophy of the Pretectal Nucleus Lentiformis Mesencephali.” Journal of comparative neurology. 500.2 (2007): 211–221. Web.

3. Troje NF, Frost BJ. Head-bobbing in pigeons: how stable is the hold phase? J Exp Biol. 2000 Mar;203(Pt 5):935-40. doi: 10.1242/jeb.203.5.935. PMID: 10667977.

4. Theunissen LM, Troje NF. Head Stabilization in the Pigeon: Role of Vision to Correct for Translational and Rotational Disturbances. Front Neurosci. 2017 Oct 5;11:551. doi: 10.3389/fnins.2017.00551. PMID: 29051726; PMCID: PMC5633612.

Cover photo by Mai Moeslund on Unsplash

Figure 1 from José Braga on Wikimedia Commons