This is a question I’ve often wondered. The way pigeons walk was the first thing that attracted me to them when I was a little girl. I love the head-bobbing and strut of a pigeon. It entertained me endlessly while I would wait for a bus to take me home after school.
Firstly, I want to explain what the so-called “head-bobbing” movement really is. It is characterised by a rapid forward movement, called the thrust phase, which is followed by a hold phase. The backward movement is in fact an illusion. As stated by Necker (2007) “the head position is kept stable with regard to the environment while the body moves continuously forward. In this way head movements during walking are characterized by a hold phase and a thrust phase.”
So in layman’s terms: as a pigeon walks it thrusts its head forward and holds it while its body walks past that point and the pigeon then thrusts its head forward again. To us this looks as if the pigeon is moving its head forward and backward as it walks.
Most birds, pigeons included, have poor stereoscopic vision or depth perception (pigeons have lateral eyes with only minor binocular overlap). So, “during the hold phase [of the head-bob] the image of the surrounding world is stabilized for a short while on the retina, which increases the time to recognize and identify objects, especially moving ones” (Necker, 2007).
I think Wedderburn (2009) put it nicely when explaining why pigeons head-bob: “…it allows them to more clearly observe their surroundings for predators. The relative head holding phase provides a more stable picture; it would be far more difficult to identify very subtle movements of a cat if the bird’s eyes were moving relative to their surroundings.
“The head bob offers another advantage to birds: since their eyes are on either side of their heads, they have little binocular overlap (where both eyes can see the same object) resulting in poor depth vision. When head-bobbing, objects further away will seem to move more compared to objects that are close-up. … This is called ‘motion parallax’ and it allows birds to judge distances more effectively.”
The head-bob is not just a pigeon thing. Other species do it too.
The Orders of head-bobbing and non-bobbing species (well known species in brackets).
Head-bobbing: Columbiformes (pigeons, doves), Galliformes (chickens, pheasants, quails, peafowl), Gruiformes (cranes, rails) and Ciconiiformes (herons, storks, ibis).
Non-bobbing: Sphenisciformes (penguins), Phoenicopteriformes (flamingos), Pelicaniformes (pelicans, cormorants), Anseriformes (ducks, geese, swans), Falconiformes (hawks, eagles, vultures), Strigiformes (owls) and Psittaciformes (parrots, cockatoos, budgerigar).
Mixed: Charadriiformes (gulls, plovers, puffins) and Passeriformes (crows, starlings, sparrows).
Please go to the following website for more details on head-bobbing. It has videos and graphs to explain head-bobbing: Head-bobbing of walking birds – a review
It is not really known yet why some species head-bob and others do not.
The following publications are an interesting read, but good luck as a few are quite long!
- Troje, Nikolaus F., and Frost, Barrie J. (2000). Head-bobbing in pigeons: How stable is the hold phase? The Journal of Experimental biology 203: 935-940. Available online: http://www.biomotionlab.ca/Text/TrojeFrost00.pdf
- Necker, Reinhold. (2007). Head-bobbing of walking birds. Journal of Comparative Physiology A 193: 1177-1183. Available online: http://www.reinhold-necker.de/Head%20bobbing%20print.pdf
- Jiménez Ortega, Laura. (2005). Avian visual perception: Interocular and intraocular transfer and head-bobbing behaviour in birds. PhD Thesis, Ruhr-University, Bochum, Germany. Available online: http://www-brs.ub.ruhr-uni-bochum.de/netahtml/HSS/Diss/JimenezOrtegaLaura/diss.pdf
Another interesting article on head-bobbing:
By Pete Wedderburn
Last updated: June 25th, 2009
When I met a specialist in biomechanics at a social function recently, I asked him if he knew the answer to a puzzle that has intrigued me for years: why do some birds bob their heads backwards and forwards as they walk? Pigeons and chickens are the best examples of this odd behaviour. Were their necks connected by sinews to their legs? What was going on?
He had no immediate answer for me, but like all good scientists, he has an appetite for knowledge and the determination to find the truth. He did some research, and this week, he sent me an email that explains what’s going on with this head-bobbing birds.
The subject was analysed by a Canadian scientist in 1978, using a high speed camera to measure the movement of a pigeon’s head, breast, wingtip and foot, when: (i) walking on the ground, (ii) when walking on a treadmill, and (iii) when being carried by a person who is walking along.
Firstly, by closely examining the bird walking on the ground, he confirmed the precise nature of the movement involved. This rhythmic action of the head bob involves a rapid forward ‘thrust’ of the head and what appears to be a slower backward movement. However, the backward movement of the head is an illusion, as the head in fact stays stationary relative to the bird’s surroundings, while the body actually ‘walks past it’. This backward moving phase would be better described as a ‘relative head-holding’ phase, where the head is held (almost) stationary relative to the bird’s surroundings.
When the pigeons walked on a treadmill (which must have taken some time to train) the head bobbing stopped, since the pigeon’s body was not moving relative to its surroundings.
When the person carried the pigeon while walking, the thrust and relative head-holding phases reappeared as the pigeon was again moving relative to its surroundings.
Other scientists took this work further, training birds to walk on the ground when blindfolded. These birds did not bob their heads, further confirming that the head bob is prompted by seeing the surrounding environment moving relative to the bird.
So why do birds use this thrust and relative head-holding action? The best guess is that it allows them to more clearly observe their surroundings for predators. The relative head holding phase provides a more stable picture; it would be far more difficult to identify very subtle movements of a cat if the bird’s eyes were moving relative to their surroundings.
This is easy to show. Ask a willing assistant to stand with a bright object in their hand (representing a predator) while you stand 10m away. See if you can detect when your assistant moves the object just a few centimetres (about 5 cm is observable). Ask them to repeat this while you are running parallel to them and you will see it is nearly impossible to identify when the bright object is being moved this small amount.
The head bob offers another advantage to birds: since their eyes are on either side of their heads, they have little binocular overlap (where both eyes can see the same object) resulting in poor depth vision. When head-bobbing, objects further away will seem to move more compared to objects that are close-up. (Try holding your finger in front of you and move your head from side to side, and you’ll see what I mean.) This is called “motion parallax” and it allows birds to judge distances more effectively.
I thought that it would impress readers if I could duplicate the scientific research in my own back garden, so last night I took some video footage of one of my own hens walking on her own, then being carried by my daughter.
It’s a good demonstration of the walking head bob, but it’s less easy to see the bob when the bird is being carried.
Scientists are willing to study anything; they are sometimes driven simply to understand, even if there appears to be no real benefit to us. And now you too know why hens use that much-mimicked head-bobbing chicken-walk.