The completely stationary field of view of pigeons (and chickens, pheasants and starlings)

The completely stationary field of view of pigeons (and chickens, pheasants and starlings)

You have to learn to love city pigeons. Spontaneous sympathy for the birds is rare, they are too close at hand, too busy and too pushy. They smear facades and monuments, foul their own nests and exhibit a horniness that can spoil the appetite. Think of the vomiting cooing of the courtship cock.

The civilized wood pigeons that the AW center feeds have suffered for years from the aggression of city pigeons until they realized that they could simply chase the animals away – wood pigeons have only small brains and are still amazed by the blowing of the wind. Yet they effortlessly balance themselves on swaying branches where the city pigeon, poor offspring of the rock pigeon, does not even dare to look.

It was always nice to praise the city pigeon until 2022 Bird Atlas Amsterdam appeared with a remarkably sympathetic view of the animal. He was not aggressive but combative, he was not a pervert but a survivor, he was au fond just as cosmopolitan as we are, with just as little feeling for nature. Wherever civilization flourished, the city pigeon flourished, wrote the Bird Atlas.

In this way, a person learns to see city pigeons with different eyes and the first thing you may notice is that they are acting strangely busy with their heads. While walking, this always jerks forwards and backwards. You’ve known that for some time, of course, but suddenly you notice that most birds don’t do it. Ducks and geese never do it. Birds that don’t walk but don’t hop either. Black-headed gulls only do it occasionally. You actually only find head movements in pigeons, chickens, pheasants, starlings, magpies and cranes. And with swimming coots.

On a treadmill without a blindfold

And truly, also with the worthy wood pigeons. In all cases, the rhythm of the head movements corresponds to that of the legs, but without evidence of a biomechanical coupling between the head and legs. The latter was demonstrated by psychologists Dunlap and Mowrer from Johns Hopkins University in elegant experiments around 1930. If you hold a pigeon or chicken in your hands and move forward at a bird’s pace, the head movements start immediately. If you let the animals walk on the ground under their own power with their eyes taped shut, their heads will remain still. If you place the animals, now without a blindfold, on a treadmill that keeps them in the same place in relation to the environment, head movements will also be prevented. So they are visual stimuli the conclusion was that they induce head movements.

The literature has started to call the phenomenon ‘head bobbing’. The interesting thing about head bobbing is that it doesn’t go the way it seems. The pigeons do not move their heads symmetrically forwards and backwards, but only forwards. The retrograde movement is an optical illusion, which was expressed as a suspicion in the American weekly in 1927 Collier’s and could be confirmed by Dunlap and Mowrer using a high-speed film camera. After the jerk in a forward direction (the thrust phase) the pigeon freezes its head in relation to the environment while its body passes underneath. That’s the hold phase.

It cannot be seen with the naked eye, but it is clearly visible in YouTube videos. Courting city pigeons in the Vondelpark or a wood pigeon walking somewhere in Europe: you freeze the video and watch it frame-by-frame by pressing the period or comma key. The hold phase immediately stands out, it is a miracle.

Dunlap and Mowrer immediately understood what head bobbing is good for: only by throwing the head forward with equal force does the pigeon have the opportunity to immediately afterwards come to a complete standstill for a short time so that the retina receives a perfect still image of the environment. So that it becomes clearer whether something is moving somewhere that could pose a danger.

Very astute

At the same time, the Hopkins psychologists saw a problem: is it not strange, they wrote, that the completely stationary field of view of a pigeon that moves neither its body nor its head does not induce bobbings and that the stationary field of view of the pigeon in hold phase does? ? They suggested the possibility that the pigeon’s head moved slightly during the hold phase.

Other researchers later called out the Hopkins concerns very astutely, and a lot of work was done to record the supposed movement. ‘How stable is the hold phase.’ Indeed, a minimum ‘slip’ has been established with high-speed cameras: in the hold phase the pigeon’s head appears to move at a speed of 3 mm/s with the forward movement of the pigeon’s body. Completeness compulsion then led researchers to investigate whether, in addition to these minimal translation perhaps also unnoticed rotations would be small head movements that the fern guest would compare to the pitching, rolling and screeching of his ship. Sexually frustrated cocks and modern video cameras were used for this, but to no avail. The ability of pigeons and chickens to keep their heads in one place under almost all conditions is legendary.

Is head bobbing now sufficiently explained? In 2009, an article was published by an international research group led by Laura Jiménez Ortega that turned everything upside down again. In a refined experimental design similar to the Skinner box, pigeons were made to respond to light images that differed only minimally from each other. It turned out that they saw the difference between the two shapes just as well in the thrust phase as in the hold phase. Dunlap and Mowrer didn’t have to experience it.




SCIENCE