Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!wuarchive!uunet!stretch.cs.mun.ca!leif!dgraham
From: dgraham@kean.ucs.mun.ca (David Graham)
Newsgroups: rec.birds
Subject: Parallax vs receptor saturation (was Re: Head Jerking)
Message-ID: <122642@kean.ucs.mun.ca>
Date: 6 Aug 90 09:30:20 GMT
References: <1990Jul28.033019.5059@ddsw1.MCS.COM> <63474@oliveb.atc.olivetti.com> <4082@trantor.harris-atd.com>
Organization: Memorial University. St.John's Nfld, Canada
Lines: 35

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. I often have marvelled at just how totally motionless the
> bird is able to hold its head as it walks. What an effective biological servo
> control system this is!
> 
I like the theory recently advanced here that it's a question of 
saturation vs refreshing of receptors in order to provide optimum 
resolution of incoming predators. But what about raptors? Perched owls 
and hawks will bob their heads from side to side, and tilt them as 
well. I've read that this is to provide best parallax prey in their 
visual field, but now I'm wondering if receptor saturation might not 
have something to do with that too (only in reverse, if you follow 
me). Anyone have any ideas? Sam? Kehaar?
 
> ______________________________________________________________________________
> Bob Davis              \\ INTERNET : sonny@trantor.harris-atd.com  |  _   _  |

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   David Graham                                 dgraham@kean.ucs.mun.ca  
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