kfir on 20/9/2007 at 01:32

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Why do birds keep moving their heads back and forth whenever they walk?

Gingerbread Man on 20/9/2007 at 01:48

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Actually, if you watch what they're doing, the head is still. It's the neck and body that bob back and forth while the head (more importantly, the EYES) stay still longer as the bird walks. Especially since what a bird is looking around for is motion. They don't have the same level of "steadicam" that we do.

demagogue on 20/9/2007 at 04:51

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Isn't this a question for Vivian?
Wasn't he the one working on the biomechanics of dinosaur walking or something like that?

Variant on 20/9/2007 at 10:56

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I think the real answer is that nobody knows for sure.

It could have to do with their vision, or balance, or just because they feel like it. It's hard to say because there are so many different behaviors in birds, especially when on the ground. Some walk with the head bob, some don't bob when they walk, some hop and others just run.

Vivian on 20/9/2007 at 12:02

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They bob their head when walking to gather parallax data for depth perception. Another, more wierd reason has to do with special bird retinal cells - they apparently have these visual systems that 'tune out' percieved backwards motion when the head is going constantly forwards, so they can tell the difference between something seeming to go backwards because they're flying past it and something actually going in the other direction when they fly past it. The theory is that the hold-and-thrust of the head bob keeps these units from getting activated during terrestrial walking, when being able to tell if things are approaching you is important if you don't want to get eaten. I dunno if thats been proved or disproved, but the parallax thing seems to be true.
They don't bob their heads on a treadmill, so balance function has been pretty well disproved.
Corvids (crows etc) have more-or-less stereoscopic vision, so they don't need to head-bob that often to gauge distance. Pigeons, on the other hand, have eyes on each side of the head, so head-bob would be very important for depth perception.

Here's an old paper on it: (http://jeb.biologists.org/cgi/content/abstract/74/1/187)

I have a pet theory that one of the reasons for having such a long neck in a lot of theropod dinosaurs is to get maximum travel on a head-bob, and hence maximum visual cues on distance. I doubt there would be a good way to try and prove it.

Oh yeah, and GBM is right - keeping an image on the retina long enough to tell what it is probably a major reason for the 'hold' bit of the head-bob.