Why Do Birds Bob Their Heads When Walking?

[DaveC426913]

Belongs in a 'nature' forum, but this is as close as it gets.

Why do some birds (pigeons, starlings) bob their heads (front and back) when they walk?

I have two hypotheses:
1] They have lower retinal retentiveness, and must hold an image on their eye longer to resolve it. They move their heads so as to spend a maximum amount of time with their eyes motionless with respect to the environment.
2] It's a centre-of-gravity balance/counterbalance thing.

 

[Moonbear]

It's not an issue of lower retinal retentiveness (I'm not even sure what you mean by that term). Birds tend to have very good visual acuity. But according to this ornithologist at Cornell, they do bob their heads to maintain their eyes in a fixed position longer to get a clearer view of what's around them. The reason they do it isn't necessarily that they need to do so any more than any other species, but simply that they can, due to the length and structure of their necks.

http://www.ccmr.cornell.edu/education/ask/?quid=472

(Hmm...I might have to keep an eye on those giraffes next time I see them to find out if this happens in non-avian species with long necks.)

 

[DocToxyn]

The gait and speed with which many birds move requires them to break up the optic flow of the environment at certain portions of the stride by stopping the head. The ensures that they avoid blurring and missing potentially dangerous or beneficial events.

Take a look at the work by H. Gioanni. There are numerous papers of his/hers on PubMed that investigate optocollic reflexes, saccades and other parameters of pigeon vision.

 

[DaveC426913]

It's not an issue of lower retinal retentiveness (I'm not even sure what you mean by that term).

Sorry, did I use the wrong term? Perhaps better known as 'image retention' or 'persistence of vision'. They refer to the length of time an image 'stays' on our retina. For humans, this is about 1/10 of a second. If it were faster, we would not be able to watch movies because we would see individual images rather than a smooth sequence. I believe cats and dogs have a faster retention speed.

My hypothesis proposed that birds had a lower retention speed which would mean that, unless they kept their head still for a larger fraction of time, they would see everything as a blur of motion.

Anyway, debunked, so OK.

Thanks.

 

[gerben]

Also it is probably beneficial for depth vision, by looking at their surroundings from different view points, you get depth information from the changing relative positions of objects at different depths (you can try it yourself, by bopping your head), this kind of depth information is probably especially important for animals that do not have a large overlap in the fields of view of both eyes (or have both eyes close together) like pigeons.

 

[Almeisan]

I heard they do it because of depth vision too. Because both eyes are on opposite sides of their head, they cannot see depth without moving their head.

 

[Moonbear]

I heard they do it because of depth vision too. Because both eyes are on opposite sides of their head, they cannot see depth without moving their head.

As far as I know, birds have no problem with depth perception. Their fields of view are pretty wide and do overlap. This would make more of a sense as an argument if they were continually turning their heads side-to-side, to get objects off to the side into the field of view of both eyes, but bobbing forward and back wouldn't help with that (we rely on parallax for depth perception, which means both eyes need to be focused on the object).

 

[gerben]

This really is the view of many researchers in the field (that the head bobbing gives them better depth information). Some birds do indeed have overlap in the fields of view in both eyes but some do not (not all birds bob their heads). Moreover, if the interocular distance is small retinal disparity is hardly detectable. Birds like pigeons with eyes on the side of their head can get good motion parallkax information by moving their head forwards-backwards, probably better than by moving side-to-side. Birds with forward pointing eyes owls often do move side-to-side.

 

[SpaceTiger]

Sorry, did I use the wrong term? Perhaps better known as 'image retention' or 'persistence of vision'. They refer to the length of time an image 'stays' on our retina. For humans, this is about 1/10 of a second.

On a side note, this is probably the same as (or very close to) the effective "exposure time" of our eyes, and explains why galaxies and planets don't look quite as nice when peering through a telescope as they do in photographs. Other detectors (like photographic plates and CCDs) can be exposed to the light for longer periods of time before forming the final image, so they can depict a lot more detail in the image.

The downside to longer exposure times (and the reason that we don't have them with our eyes) is that it decreases your ability to detect motion. If you're trying to hunt deer, you're much better off getting a mediocre quality image every 1/10 of a second than a high quality image every second or 10 seconds. Otherwise, by the time your eye was finished exposing, the deer would just be a blur across your field of vision. This same problem occurs in photographing galaxies if we don't keep moving the telescope. Why? Cause the dang Earth keeps rotating and changing the position of the object!