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From: mjm@oliven.olivetti.com (Michael Mammoser)
Newsgroups: rec.birds
Subject: Re: HEAD JERKING OF WALKING BIRD
Message-ID: <49252@olivea.atc.olivetti.com>
Date: 9 Aug 90 05:03:18 GMT
References: <1990Jul28.033019.5059@ddsw1.MCS.COM> <63474@oliveb.atc.olivetti.com> <4082@trantor.harris-atd.com>
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In article <4082@trantor.harris-atd.com>, sonny@charybdis.harris-atd.com (Bob Davis) writes:
> In article <63474@oliveb.atc.olivetti.com> mjm@oliven.olivetti.com (Michael Mammoser) writes:
> >
> >This difference (parallax) is the way that we make accurate
> >judgements of the distance that the object is at. Birds with 
> >monocular vision are viewing a distant object with only one eye.
> >They bob their heads back and forth to shift the position of
> >their eye in order to provide this parallax. Sounds logical anyway.
> >
> >Mike
> > 
> NO, NO, NO. As another poster to this thread has already
> accurately observed, THE HEAD IS RENDERED ABSOLUTELY IMMOBILE 
> RELATIVE TO THE EARTH by the bird's "head bobbing" motion as it walks. 
> Look at it closely. 

	NO, NO, NO! The bird can hold its head absolutely immobile
only in so far as it can stretch its neck. Once the neck is stretched
as far as it can go, if the bird continues walking, the head MUST
become mobile. Actually, the poster was differentiating between the
bird continuously moving its head back and forth while it stepped,
and holding the head stationary while it stepped and moving it
forward at the end of the step. No matter what the method, it has
no bearing on the idea that I put forth. However, here are some 
references that I found concerning head bobbing.
 
Bent's Life Histories of North American Gallinaceous Birds

	On the Key West Quail-Dove, Audubon (1840) writes:

	"...and it moves its neck to and fro, forward and backward,
as pigeons are wont to do."

Bent's Life Histories of North American Blackbirds

	"The gait of the Brewer's Blackbird is usually a walk,
accompanied by short forward jerks of the head. When the bird runs
there is no jerk. Mulford (1936) outlines the process of taking
two walking steps as follows: '1. Head is thrust forward as one leg
and foot are lifted. This moves the center of balance forward.
2. Leg and foot are brought forward. 3. Head is pulled back as body
is brought forward by step and as foot is set down on ground.
4. The sequence of movements is repeated with other foot.' "


	On 30 Jul 90, Pat Hertel writes:

"I believe that the head jerking is due to the fact that a bird's eye
is fixed and therefore it has to move its head to "refresh" its
receptors."

	On 30 Jul 90, Gregory Bloom writes:

"I believe that birds eyes do not constantly wiggle to avoid saturating
the receptors (eye wiggle == nystigmus?) (sp), so when their heads are
still, images that don't move will quickly 'bleach-out' the receptors
they fall on."

	On 1 Aug 90, Keith Lofstrom writes:

"Birds have small brains, and can't distinguish between their own
motion and the motion of the predators they are trying to avoid."

	Now none of these gentlemen site any literary references
for the views expressed, so I don't know if these are accepted
theories of ornithology or not. However, I found the following
reference in "Watching Birds, an Introduction to Ornithology":
 	
	"The size of various parts of a bird's brain reflects the
relative importance of the areas they control. The optic lobes,
related to vision, are large."

	"Birds can distinguish objects much farther away than can
humans, and their vision is in fact the most highly developed of
any animal."

	Now, it seems strange to me that a bird, with the most 
highly developed vision in the animal world, would have a problem
"refreshing" its receptors (the same rods and cones that we have),
or having them "bleached-out", or distinguishing their own movement
from that of an external object. One final note from "Watching Birds,
an Introduction to Ornithology":

	"Most of the area a bird sees is perceived with only one
eye... A disadvantage to perceiving an object through only one eye
is the difficulty in judging distance, and for this reason you often
see a bird cock its head to carefully focus on an object; likewise,
the bobbing up and down of the head characteristic of many shorebirds
may be an effort to guage distances."

Mike