Why do birds walk funny?


29 mei 2019
Auteur(s): Philip Lepoutre
What is surprising are not the hypotheses themselves, but rather the methods through which they were tested. Birds do walk funny...

by Philip Lepoutre

Science Editor

Evolution as we know it, first proposed by Charles Darwin in his 1859 book, “The Origin of Species”, has shaped organisms around us in very strange ways. I’ve been scratching my head lately as to why evolution made flightless birds with wings, or made snails that are able to change their gender at will. However, if we shift our interest to birds, we can see there’s something very curious in the way pigeons and chickens walk. In this article, we will explore the mysteries of birds’ funny ways of walking.

As a matter of fact, not only do pigeons appear to bob their heads when they walk but so do chickens, doves, and cranes, among other birds.

While some theories propose that birds bob their heads in order to help maintain their balance or it provides improved depth perception, most scientists agree that the most likely reason is to maintain an acute sense of their surroundings. Since chickens and other small birds are hunted by predators, it’s important that they constantly scan the area for dangers while they walk. And since they feed off small bugs and insects, it’s vital that they are able to sense movement around them as they walk.

How did scientists conclude that this behaviour isn’t a response for keeping their balance?
Strangely enough, Dr. Frost and his team investigated this issue by starting from two hypotheses: “one which supposed the head movements were primarily to maintain the equilibrium of the bird during walking; and alternatively that it was primarily visual response involving periods of stabilization of the bird’s visual world,” said Dr. Frost. What is surprising are not the hypotheses themselves, but rather the methods through which they were tested. They put a pigeon, encased in a glass box, on a treadmill running at an average walking speed. To summarise the lengthy research that they published under the name “The Optokinetic Basis of Head-Bobbing in the Pigeon”, Dr. Frost and his team found that “if the bird’s walking velocity matched the belt velocity then no head-bobbing movements were produced.”

Head, breast and foot position as a function of time for a pigeon walking on a treadmill. As can be seen, the head movements have been practically eliminated.

Notice how the head barely moves through the entire time period. This is a clear indication that if the environment doesn’t change, the head-bopping disappears. This is precisely what led scientists to conclude that this funny walking observed in certain bird species is led by an instinctive reflex of wanting to adapt to new environments.

Unlike humans, these birds don’t have eyes that move in an eye socket, meaning that they’re fixed. So in order to shift their gaze, chickens, among other birds, have to move their entire head and if they choose to focus on an object, they have to keep their head still (even if their body may be in motion). According to Frost’s experiment, “the percentage of time the head is stationary during walking is approximately 63%”.

This behaviour is one of many wonders nature has in store for these animals; by keeping their head steady (even if it is for a mere 20ms), their eyes have the opportunity to focus their vision on their surroundings and track nearby movements while they are on the move.

This incredible image stabilisation tool allows birds to spot the bugs they want to eat while they are on the run, as they make use of a stationary background to detect even the slightest movements. Even though we may believe our eyesight to be the most complex mechanism known to life, we have proven envious of this image stabilisation. Among many other proofs, I present to you the “Mercedes-Benz Chicken MAGIC BODY” TV commercial, which really highlights just how stable a chicken can keep its head, even as its body is moving. I highly suggest you go check it out on YouTube.

It really is fascinating how a chicken can keep its head still despite its body moving, and people from all corners of the internet have tried playing around with birds and testing their innate skills for their head stabilisation.

But, is it really bopping? In fact, it turns out that there is no backward, up or down movement at all. A bird like a chicken or a pigeon will move its head forward, then lock its head (therefore its eyes) in place, allowing its vision to stabilize and give their eyes enough time to scan the peripherals for any danger or prey.