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From: andrewt@cs.su.oz (Andrew Taylor)
Newsgroups: rec.birds
Subject: Re: HEAD JERKING OF WALKING BIRD
Message-ID: <1145@cluster.cs.su.oz>
Date: 11 Aug 90 09:55:57 GMT
References: <1990Jul28.033019.5059@ddsw1.MCS.COM> <63474@oliveb.atc.olivetti.com> <4082@trantor.harris-atd.com> <49252@olivea.atc.olivetti.com>
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Reply-To: andrewt@cluster.cs.su.oz (Andrew Taylor)
Organization: Basser Dept of Computer Science, University of Sydney, Australia
Lines: 23

From "The Life of Birds" by Welty:

"Doves, gallinaceous birds and rails normally accompany each walking step
with a sudden forward jerk of the head. The head then remains fixed in
space as the body moves forward until the next step is taken. This
permits sharper vision than would otherwise be possible, for a steadily 
moving head can not see objects - expecially other moving objects -
as clearly as can a head fixed in space (Stresemann, 1927-1934).

Motion picture analyses by Bangert(1960) of the head and leg movents of the
the domestic Chicken, Gallus gallus, showed that the two movements are
strictly co-orddibated and in a rhythm of a about 2.2 to 3 cycles per second.
If a bird is passively carried foward legs free from the ground, so that
its retinal images will shift, the head will bob to and fro, along with 
perfectly coordinated leg movements. if the stimulus of shifting retinal images
is prevented by ground-glass spectacles, the head still swings to and fro
whenever the bird walks. The innate nature of both head and leg movements
was established with newly hatched chicks reared in compleete darkness."

I'd recommend The Life of Birds to anyone interested in a gaining a deeper
knowledge about birds. I believe its commonly used as a university textbook.

Andrew