It’s all down to good breeding and deportment. GodlessHeathens
If you believe pigeons are the most severe example of this behaviour, try chickens. They crane, rotate, jerk and generally goof their necks around even when not walking. I always understood that this is because birds have eyes facing different directions, and so can’t take advantage of the differential we use for depth perception. As keen predators, chickens have to make significant snap judgments about the speed and distance of a target (be it insect, mouse, lizard or indeed something wholly inappropriate such as the neighbour’s cat). Pigeons are not predators, but neither are they ground birds as our modern pests are descended from rock pigeons, so who knows. I would put geese alongside your ducks in birds that don’t jerk as they walk. In my experience, they are usually too busy craning their necks forward and hissing at my ankles. However, they are both grazers or water feeders, so maybe that is it. A parrot will sit on a branch and crane from side to side. Birds are bizarre, and you should not question their motives, lest they destroy you. Lewis Barton
Weight shift, mainly. For some birds their legs carry messages, they hear through them into the ground. You could say they have X-ray legs. They can run with alacrity and stop on a dime. For other birds, walking is a more arduous business; more like waddling. Their centre of gravity is further back and head movements compensate for this. jackreader
Birds have evolved to use head motion much more than eye motion in order to view their surroundings. When moving, a bird stabilises its head in space in order to watch its surroundings, and then pops its head forward and stabilises again – it looks like bobbing, but actually it’s more like fixed-jump, fixed-jump. That contrasts with a human, say, who can use eye motion to scan the surroundings or to fixate while walking. That’s why some birds show the bobbing motion. As for why ducks are different, my guess would be that ducks evolved to be mostly on the water for safety, and it wasn’t so important to have good scanning ability for predators while walking on land. I just did a quick check and a crow’s head has a bobbing motion, but not a swan’s head. For what this is worth. hyperpeeb
Heads move back and forward so that a bird can get stereo vision by having two different viewpoints. This happens when the bird’s eyes are not both able to see the same object within their angle of view. We are lucky that our angle of view is very large (more than 180 degrees) and so we have natural stereo vision. doge
If you ever handle a bird skull, they are light because flying demands it, and have big eye sockets because they nearly all use sight to feed themselves. To fit those large eyes in, bird eye sockets don’t allow rotation to anything like the same extent as mammal eye sockets.
Having to move the whole head to change view means the head has to stay still while looking. Some birds are uncanny the way they hold the head still while the body moves; try to find footage of kestrels hovering.
Walking and ground feeding, as pigeons do, sees them use jerky head movements to focus on one view at a time. Smaller ground feeding birds such as blackbirds tend to hop and pause. The smallest birds, such as tits, don’t tend to walk on the ground at all, preferring to hop from perch to perch. Larger birds such as herons use a much slower move and pause to stalk prey, but watch how they move between static head positions.
When a duck walks on land, it is, unlike a pigeon, not habitually searching the ground for food. And duck legs are evolved primarily for swimming, so moving forward on land the dominant gait is rolling side to side. Sudden head movements would make that difficult, so if they need to focus they pause rather than coordinate head movements with steps like a pigeon.
I understand that birds do not combine the sight from their two eyes into one view, as we do. The sight from each remains on the side of the brain it feeds. Does this explain the way that starlings fly in huge flocks as murmurations? Each bird is very aware of their immediate neighbour, but not aware of the whole context. Therefore, they react to one another, and this is combined into a major collective action. Is this evident in other species? If it’s only starlings, why just them? BigBear2
I have always thought, without researching it, that birds whose heads bob backwards and forwards are ground feeders. If you watch them, the two movements are not of equal speed. The head moves back at exactly the same speed as their body moves forward, and so remains static relative to the ground, giving the best opportunity to see small grains, beetles or whatever. The head then snaps forward to the next position. From the bird’s point of view, it’s almost as if it’s taking a series of snapshots of what is ahead, each from a couple of steps forward to the last. No idea if this is the answer. Prudenceman
The American robin – in the thrush family – is a ground feeder, but doesn’t do this. They sort of tilt their whole body forward and run. WTobiasJr
I hate to say this, but you can discover why just by Googling: “Why do birds nod their heads when they walk?” As for “most”, I believe only eight of the 27 bird families exhibit this behaviour. gefreiter
I think DuckDuckGo would be a more appropriate search engine. Hagbard_Celine