Here’s an experiment: While reading this sentence, shake your head back and forth, and then nod up and down. Notice how the words on your screen remain in focus? Congratulations, you just experienced “gaze stabilization.”
It's this natural ability that allows us to maintain a clear line of vision even when our bodies are in motion. Although modern humans are largely sedentary, the capability has considerable value (how else would you read your texts while walking?). But for birds, many of which zip through trees and divebomb prey at high speeds, gaze stabilization is indispensable, fascinatingly complex, and, for those watching, even a little amusing at times.
When you shook and nodded your head at the start of this article, your eyes naturally stabilized the image in front of you. Birds use this approach, as well; they are capable of making micro adjustments in their eyes. But they do so to a much lesser extent than us. What makes birds the true steadicams of nature is how they orient their head. Take a look at the clip of the Common Kingfisher above, which aired on the BBC's Winterwatch program in January and has since made its way around the internet (on the social media platform Reddit, the clip has been upvoted more than 120,000 times).
Thanks in part to a large number of vertebrae and muscles in their neck, birds can hold their head in place even when their body’s in motion. According to David Lentink, an assistant professor at Stanford University who studies biological flight, this adaptation helps birds stream high-resolution visuals when moving quickly through complex terrain. “They keep their head absolutely horizontal at all cost because that way they have the most reliable information, which they have to stream at high rates,” he says. “When you’re maneuvering like crazy . . . you need a perfect vision platform.”
By keeping their head steady, birds like the kingfisher don’t have to rely on their eyes alone for image stabilization, says David Dickman, a neuroscience professor at Baylor College of Medicine. “The branch is blowing all over the place, so if the bird’s head wasn’t locked to gravity and stable in space, then they would constantly have to be moving the eyes to maintain [focus].” And depending largely on eye movement to control stabilization, as many vertebrates do, is not advantageous for birds, Lentink says. When birds are in high-intensity motion, their heads become a stable platform. Large movement in the eyes takes away from that stability and thus reduces the accuracy of visual information. “You don’t want to have moving eyes,” he says.
Gaze stabilization in birds is complex and still poorly understood, Lentink says. But what we do know is that it’s controlled in part by visual input and the vestibular system, a sensory system in the inner ear in charge of balance and spatial orientation. Like a three-dimensional level, the vestibular system (which other vertebrates have, as well) governs spatial orientation, and it’s critical for keeping the head steady. “The vestibular system is making sure that the head has the information to stay absolutely at the same point so the eyes can stay focused, which is really important in the kingfisher ” Lentink says.
The kingfisher in this video is a prime example of how this ability can benefit birds that hunt, allowing them to remain locked on their prey whether on a swaying limb or hurtling through the sky. But even birds from the farm show a talent for gaze stabilization. In a 2013 Mercedes-Benz commercial, chickens with steady heads but bodies in motion were used to advertise the company’s “Magic Body Control” suspension.
Pigeons also clearly exhibit gaze stabilization, says Dickman, who's been studying the city birds since 1984. When a pigeon takes a step forward, it holds its head in place so as to maintain focus on something ahead—french fries, perhaps. The pigeon’s head then needs to catch up with it’s body, which is now slightly offset. So in one quick motion it jerks it’s head forward, losing only a split second to blurry vision (which the brain ignores anyway, Dickman says).
Gaze stabilization found in nature has inspired modern technologies, such as those used to stabilize images in cameras and drones. "Any cameras that films movies have image stabilization," Dickman says. The BBC's Planet Earth II, for example, owes its stunning shots—many of which were filmed from a turbulent helicopter—in part to image stabilizers. But despite our best attempts, birds still outperform our own inventions, Lentink says. “If you hold a stabilization system in your hand, it’s relatively limited,” he says. “It’s not a fast system compared to birds.”
Yet another reminder that it's hard to improve upon nature.