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From: gregory@agcsun.UUCP (Gregory Bloom)
Newsgroups: rec.birds,sci.misc,sci.bio
Subject: Re: HEAD JERKING OF WALKING BIRDS
Message-ID: <821@agcsun.UUCP>
Date: 30 Jul 90 19:31:01 GMT
References: <1990Jul25.021150.14574@ddsw1.MCS.COM> <1990Jul26.181953.17452@ccu.umanitoba.ca>
Reply-To: gregory@agcsun.UUCP (Gregory Bloom)
Organization: Ampex VSD Golden Engineering, Golden, CO
Lines: 25

:1144

In article <1990Jul26.181953.17452@ccu.umanitoba.ca> yackob@eeserv.ee.umanitoba.ca (Kerry Yackoboski) writes:
>In article <1990Jul25.021150.14574@ddsw1.MCS.COM> arf@ddsw1.MCS.COM (Jack Schmidling) writes:
>>              HEAD JERKING OF WALKING BIRDS
>> 
>> There were a number of theories as to why walking birds move 
>> their head forward and back in synch with footsteps....
>
>	Might it be that the bird sees motion better than still objects,
>and when it is walking forward (or when you are walking it) it wants
>maximum vision?  Could it be an attempt to emulate Chuck? :-)
>

I believe that birds eyes do not constantly wiggle to avoid saturating 
the receptors (eye wiggle == nystigmus?), so when their heads are 
still, images that don't move will quickly 'bleach-out' the receptors
they fall on.  This means that when a birds head is still, it only sees
things that are moving, which is probably a real good way to spot
incoming predators and worms and such against a complex background.
The way a bird can maximize this sort of motion detection is by
moving its head in quick jerks as it walks, giving about an 80% 
duty cycle to frozen-eyed motion detection.  


Gregory Bloom
agcsun!gregory@boulder.colorado.edu