Why do pigeons bob their heads?

19 Comments
Posted November 23rd, 2008 in Biology. Tags: , , .

Recently, I’ve started to wonder why pigeons bob their heads in such a violent manner when they walk. This habit seems like it consumes a lot of calories… don’t they need every bit of energy in order to fly? I don’t understand why natural selection hasn’t bred pigeons that walk without bobbing their heads. Wouldn’t they be more efficient and therefore more fit?

Here are some hypotheses I’m toying with to explain this behavior:

  1. Maybe this actually is the most efficient way for pigeons to walk. Perhaps they’re counterbalancing themselves the way a cat does with its tail.

    But, if pigeons bob their heads when they’re standing still, this hypothesis wouldn’t explain why they do that. I haven’t been able to watch pigeons standing still long enough to see if they continue to bob their heads, though. (And even if they do, I guess it could be an instinct that’s only useful when they’re walking, but it’s too ingrained for them to stop when they’re just standing around…)

  2. Because they’re not predators, pigeons’ eyes are set far apart on their heads. This gives them a wide field of view which helps them to see approaching hawks. As a side effect, their binocular vision is quite narrow (anyone know how narrow?). This means that both of their eyes can only focus on objects directly in front of them. Objects that are slightly off to the side can only be viewed by one eye, impairing depth perception.

    I was standing on the sidewalk one day watching a pigeon bob its head when I decided to close one eye and see how my depth perception was affected. I still had to focus my eye’s lens to see an object clearly, so my depth perception wasn’t completely gone, but it was greatly diminished. Then I tried to mimic the pigeon. If I bobbed my head towards the object and then away, I got a better sense of how far away that object was. Closer objects appeared to get bigger when I bobbed towards them, while objects farther away didn’t.

    (Now that I think about it, I probably looked a little odd doing that…)

    Anyway, maybe pigeons are compensating for their poor binocular vision in a similar manner. Their quick motions could be an attempt to gauge the distance to the nearest scrap of food so they can accurately peck at it.

Does anyone have a better idea?

Update: Dae presented video evidence of a more compelling explanation.

Last modified February 6th, 2012
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19 Responses to “Why do pigeons bob their heads?”

  1. I’ve heard that it is to stabilize their vision. Maybe if they didn’t bob their head it would just kind of bounce around as they waked and they’d get a shaky view of the world. Like riding a bike on a bumpy road. Depth perception could also play a role.

    As for the the not being the most efficient way to do things remember that nature doesn’t necessarily always design the most efficient things. It makes things that get the job done. For pigeons I guess bobbing is good enough.

    -Roy

    P.S. Found your site through the project wonderful ad you put on my site. I like it and I’m subscribing to it

    • Thanks! Looks like those RSS links aren’t useless after all…

      Interesting idea. I’m not sure I get it, though. Doesn’t the bobbing motion make a pigeon’s view even shakier?

      If pigeons were trying to stabilize their vision I think they’d use their necks like a suspension system to keep their heads stationary. It seems like their heads would move around a lot less if they just walked without bobbing their heads like humans and other bipeds do. Their necks are certainly strong enough– that motion looks so abrupt that it sloshes their brains around!

      And you’re right to say that natural selection only produces local maxima, not perfectly optimized creatures. I guess I’m just fascinated by the inefficiencies. I’m curious to see whether they’re genuinely non-optimal, or if I’ve missed some crucial detail.

      • To tell you the truth I never really thought of why bobbing would stabilize their vision. After walking in front of a mirror a couple of times i thought maybe it was because when we walk our upper body more or less stays straight and this wasnt the case for pigeons. Maybe their body moves substantially enough that a significant bobbing is required to keep their head moving forward at a constant rate like ours do without having to bob.

        However after watching a couple videos on youtube of pigeons walking (this is instead of studying for my microwave ad RF circuits exam btw) i’m ready to throw out this theory.

        Then the theory of maybe when they walk their body blocks out their vision ability to see behind them (which I assume they have to a certain extent at least due to their eye position)and bobbing forward would allow them to see around their shoulders. After watching more pigeon videos I dont think their heads are low enough while walking for the shoulders to block any vision.

        I’m tempted to google this, im sure the answer is out there, but i kind of want to see if I can figure it out myself.

        hmm after skimming by the rest of the comments it seems theres some pretty heavy theories going on there, ill have to check them out when I have more time

    • Reythia posted on 2008-11-24 at 10:10

      I don’t have an answer for you, DS (I could nickname you “Dumb”, but that would be mean!), but whatever it is, it’s engrained in chickens too, if I recall. Probably other birds, too. So its evolution must go back a ways. As you say, it might be a case of a “leftover” trait — sort of like the human tail bone.

      Roy, I don’t have any particular knowledge about birds or pigeons in particular, but I know that in humans, the brain does an impressive amount of tracking to account for the bouncing of our heads. This means that when you nod your head or jump up and down, you don’t immediately get dizzy or lose your place visually. The brain is capable of calculating the effect the angle of your head and placement of your body have on your vision, and is able to “stabilize” the results for you so everything doesn’t end up hopelessly blurred. (The human brain is a miraculous machine — it takes a LOT of math and coding to get a satellite to do approximately the same thing!)

      Anyhow, I would guess that birds have much the same ability (though I could be wrong). If they do, then the bobbing of their heads would have no effect on the “bounciness” of their vision at all — and neither would a wobbly neck.

      Even if I’m right about the stability question, it’s still possible that DS’s pseudo-stereo vision theory is correct, however.

      • Bah, I chose the pseudonym. My skin ain’t that thin!

        DS’ll do fine though. Less typing = good.

        Now that you mention it, the trait does seem present in lots of birds. Depending on how widespread it is… I wonder if it could be a trait leftover from the dinosaurs?

        I have to chuckle at the idea of a Tyrannosaur bobbing its head like a pigeon, though!

  2. I found a different explanation here (look for Chris’s post).

    He says that pigeons are actually trying to minimize the time their heads are in motion. They do this by walking forward while keeping their heads in the same position it was in during their last step, then jerking their heads forward once the step finishes.

    The reason pigeons are trying to minimize the time their heads are moving is that a pigeon’s widely separated eyes leaves them susceptible to parallax distortion effects. He says that the parallax effect is especially bad when their head’s moving.

    Notice that while his explanation’s closely related to mine, it’s subtly different. In my hypothesis, pigeons are using their quick motions to effectively fake having stereoscopic vision. They’re using images from the same eye at two points in time to provide depth perception. In his hypothesis, pigeons are simply trying to avoid confusing themselves by minimizing the amount of time that they have to watch the world through eyes that are moving relative to the ground.

    I’m not sure I understand Chris’s explanation, though. I closed one of my eyes and crawled around with my head close to the floor. I wasn’t more confused when I my head was in motion; in fact as I noted earlier it seemed easier to judge distances when I moved (but only if I moved relatively quickly towards the object in question).

    Regardless, it is possible to experimentally distinguish between these two hypotheses. Chris’s hypothesis requires that the head bobbing motion be synchronized with the pigeon’s footsteps, because otherwise it wouldn’t minimize the amount of time the pigeon’s head is in motion. My hypothesis requires no such synchronization, and presumably high-speed photography could falsify Chris’s explanation. On the other hand, it’s harder to falsify my hypothesis because the synchronization could be an accidental habit rather than being strictly necessary.

    • Dae posted on 2008-12-07 at 22:02

      Your crawling around with one eye test isn’t at all the same. You’re not confused by that, because you only have one forward view of the world whether you have one eye open or both. They track on the same thing. If your two eyes could not be pointed in the same direction but there was still some overlap between their visual fields it would be a lot more confusing. In fact it is more confusing for those with eye muscle disorders that cause them to be walleyed.

      Chickens do exactly the same style head bobbing as pigeons, and here’s a great vid showing how incredibly still they can keep their heads while their body moves around.

      It’s clearly so they can stabilize their view of the world. Their necks are longer so they bob their heads at a bit lower frequency than pigeons while walking. Here’s a vid of a pigeon walking slowly, and a chicken.

      I think if you played it frame by frame, you’d see their head hold perfectly still relative to the background for a moment, then a quick jump forward, and repeat.

      Check out this odd one: Mating dance of the Prairie Chickens

      If you watch closely you’ll see that sometimes they bob their heads and sometimes they don’t. At one point you see one moving forward toward the camera without bobbing and it looks drunk, weaving back and forth. It’s possible they can’t hold their head still with their air sacks puffed up. Near the end you see two facing each other, one puffed up and not bobbing its head, and the other clearly watching it with head held still.

      From an evolutionary point of view it’s probably just that chickens and pigeons don’t really need the visual processing brain power to handle watching stuff while they’re on the move. They spend most of their time on the ground and if they hold their head still, then anything bigger than them that is moving, is a possible threat, and anything very small and moving (or not moving) is edible. They’re not predators that need to track, chase, catch and kill something while running or flying at full speed.

      • Wow, thanks! That first video alone convinced me that you, Roy and Chris are right. It’s clearly a mechanism devoted to keeping their heads stationary. (I don’t really notice the effect as clearly in the other videos, though.)

        I hadn’t really thought about the effect my binocular tracking system would have. I also didn’t realize that birds’ eyes don’t track in the same manner, but I guess I should have. Birds must see the world through two different “windows” which move independently of each other. Weird.

        I think I see what you mean now. A bird’s eyes don’t focus on the same object, so the bird’s visual cortex can’t easily use the parallax effect that humans use for stereoscopic triangulation. The problem isn’t that the overlap between its eyes is too small (which is what I thought), but rather that the overlap is too big. Whenever the bird’s head moves, the scenery in the overlapping region changes differently for each eye. Since that motion takes place in each eye’s peripheral vision, it was probably difficult for evolution to reconcile these viewpoints “in software.” Instead, a crude patch evolved: birds constantly waste a bit of energy keeping their heads stationary. Fascinating!

        Oh, and you’re right: my earlier test was ill-conceived. Thanks again.

  3. I’m confused again. This time I’m thinking about antelopes and rabbits who also have eyes on opposite sides of their heads. I don’t know if their eyes move independently like a pigeon’s, but if so then why don’t they bob their heads like pigeons?

    • Dae posted on 2009-02-23 at 00:04

      If I had to speculate, I’d still say visual processing needs. You don’t need to hold your head still to spot a tuft of grass or leaves to eat, but do need to do so to peck at a tiny bug or a seed. Rabbits and antelope also probably depend on their senses of smell to find food, and hearing to identify potential threads. Fundamentally though, I don’t know exactly why some birds hold their heads still like that, but they do, and I think it’s very clear that the bobbing comes from their need to hold their head still.

      • I’m now convinced that you’re right– birds are trying to hold their heads still. I’m not even sure that my comparison is valid because mammals might not have the same independent eye tracking system as birds. Although, if an animal with widely spaced eyes doesn’t move their eyes independently, that’d seem to negate much of the advantage of a wide field of view.

        I guess I’m just wondering if other animals with independently tracking eyes have found a different way to solve their parallax problems, or if they’ve grown to enjoy the parallax shifts (a speculation I consider wholly unsatisfactory).

      • Here’s a reference to a study performed by Dr. Barrie J. Frost where pigeons were placed on a treadmill. When the pigeons walked on the treadmill their heads didn’t bob, supporting the idea that the motion’s purpose is to keep their heads still.

  4. Tim posted on 2009-04-30 at 15:14

    Nothing to do with efficiency. The tendons in their legs are attached to the back of their heads so they have no choice. In the same way, we can’t bend the pinky without the ring finger following.

    • Interesting… do you have a reference for that statement? It seems to imply that Dr. Frost’s 1978 paper was wrong to say “The fact that head-bobbing is abolished when pigeons walk on a treadmill suggests it is primarily a visual response rather than an equilibratory response.”

    • Phafin posted on 2010-10-16 at 14:36

      Um, yes I can… Am I abnormal? What I can’t do is place my thumb, 1st, 3rd and 4th fingers on a table with my 2nd finger bent under so the 2nd knuckle is also on the table, then lift my 3rd (ring) finger off the table. Its quite frustrating really!

      As for the birds, I think it is probably a combination of all three ideas. Holding still to “resolve” the image, moving quickly to compare the image (parallax) and to aid in balance. I actually found this page after watching a boy pigeon chase a girl pigeon hooting his little heart out. She was MUCH more stable in her movements than he was and he had to stop his head bobbing and hooting to walk down the fence after her. Then she flew off and he stood there watching me for a few moments, jerking his head around like a, well, like a pigeon!

      As to what came first, the holding it still or the jerking for parallax, I don’t think we will ever know. It is a strange quirk of nature that if you can use one tool for many jobs then you probably will!

      Also, the mammals which you mentioned are land based, and therefore do not need the light weight structure of a bird. I can tell you from my hunting trips that a pigeon’s skull is cardboard thin, while a rabbit has a surprisingly dense skull. Maybe the extra weight in the brain for the neurons which do the imaging is less efficient than the wasted energy with the bobbing?

      I do know that the human brain is so big because of our imagination. We actually only process about 3-5% of what we see when in a familiar environment. Up to a maximum of 40-50% when in a strange environment. Our big brains can imagine what’s in the space between the dots faster than our eyes can send it in. It is a massive waste of energy though, so I wonder why we do? It’s not like we have the best vision of any animal, nor the widest spectrum, and we can only see to 35Hz or so!

      • Excellent feedback, thanks. The only thing I have to say is that I’m an avid computer gamer, and I can tell the difference between a 60Hz refresh rate and a 100Hz refresh rate, as well as tell the difference between a game that runs at 30 frames per second as opposed to 60. Anything above that looks perfectly smooth though.

  5. Homer Berkowitz posted on 2012-05-13 at 13:01

    To answer your question about a pigeon’s field of view: Pigeons have a field of view of 340 degrees with a binocular view of only 24 degrees.

    I read a number of different hypotheses as to why these birds Bob their heads when they walk, including the experiment on the treadmill. But I still can’t figure out exactly why they do it!

  6. Twen posted on 2012-06-15 at 18:56

    Apparently albino rats will also bob or weave their head before they make a jump, as they are known to have poor vision and this helps them judge distance better.

  7. A friend just pointed out that ballet dancers use a technique called spotting to reduce dizziness during spins. This involves staring at a particular spot in the room as long as possible, then turning one’s head very quickly to stare at the same spot again.

    This technique seems very similar to a pigeon’s head bobbing.

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