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From: mjm@oliven.olivetti.com (Michael Mammoser)
Newsgroups: rec.birds,sci.bio
Subject: Re: HEAD JERKING OF WALKING BIRD
Message-ID: <63474@oliveb.atc.olivetti.com>
Date: 5 Aug 90 21:24:57 GMT
References: <1990Jul28.033019.5059@ddsw1.MCS.COM>
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In article <1990Jul28.033019.5059@ddsw1.MCS.COM>, arf@ddsw1.MCS.COM (Jack Schmidling) writes:
> 
>  I fully accept the criminal nature of the act which I 
>  implicated myself in, but having watched a kestral snatch 
>  the first two fledglings off the nest in my bathoom window,  
>  I was in no mood to concern myself with the letter of the 
>  law.

	What was it that you thought you were accomplishing?
It seems that there are two possible consequences of your action.
You deprive a kestrel of a meal and: (a) the kestrel, not being
able to nourish itself, dies. Now you have a dead kestrel rather
than a dead dove. Do you place an intrinsically higher value on
doves than kestrels? (b) the kestrel goes off and takes some other
bird. Now you have some other bird dead rather than a dead dove.
Are doves worth more than some other species? What if the other
bird taken was also a dove? Just what were you accomplishing?


>  One responder suggested that the behavior was an adjustment 
>  for continually changing focal distance.  This seems like a 
>  reasonable answer but I can't help but wonder why these 
>  birds have such a limited depth of field.  One assumes that 
>  it must be on the order of 1/2 inch because that amount of 
>  head movement seems to put things right again.

	One of the best theories that I've heard is that the
bird does this to provide the parallax necessary to make
distance judgements. Animals with binocular vision, such as
people, view objects with both eyes at the same time. Since the
eyes are separated by some distance, the position of the object
with respect to the distant background is different in each eye.
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