NetNews Usenet Archive 1992 #16/NN_1992_16.iso/spool/sci/physics/11510

Internet Message Format | 1992-07-23 | 3.2 KB

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From: [e-mail] (Dalcio Dacol)
Newsgroups: sci.physics
Subject: Re: Blue Sky
Message-ID: <[e-mail]>
Date: 23 Jul 92 22:39:10 GMT
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In article <[e-mail]> [e-mail] (A Colin Morton) writes:
>In article <[e-mail]> [e-mail] (Barry Merriman) writes:
>>In article <[e-mail]> [e-mail] (Hal Lillywhite)
>>writes:
>>> In article <[e-mail]> [e-mail] (Jim Carr) writes:
>>>
>>> ....
>>> >George did that routine before blue-corn became popular enough to
>>> >show up as corn flakes as well as tortillas, or squid-ink pasta made
>>> >it into american grocery stores.  Interesting that there are blue birds
>>> >and flowers but not blue animals; where is a biologist when we need one!
>>>
>>> And I find it peculiar that although there are blue birds, there are
>>> no blue raptors that I know of.  Can you imagine the advantage a sky-
>>> blue hawk would have?  His prey would have a very difficult time seeing
>>> him until it was too late.  I wonder why such an obvious advantage
>>> seems not to have developed.
>>
>>Actually, he should be blue on the bottom and dark on top,
>>so that he ends up blending in when viewed from above or below.
>>Thats "why" fish are silver on the bottom and dark on top, perhaps
>>
>Most raptors are, in fact, white or very light gray on their undersides,
>with mottled brown top coats.  In most parts of the world inhabited by
>hawks and the like, partly cloudy or hazy days far outnumber the
>perfectly clear ones, so this actually works better than a blue
>underside would -- white shows up less distinctly on a blue background
>than vice versa.
>
>Colin.

    The point, it seems to me, is that any bird, no matter what
color, when seen from below against the day-time sky is going to look
like a darkish sillouete. since it is mostly backlit. The brownish-gray
color does offer some camouflage advantage because in the final
approach, hawks that hunt in open areas come in at very shallow grazing
angles and so the bcakground, frrom the prey's point of view is either
a terrestrial landscape (low hills, bushes, etc) or the distant
horizon, which in general is not blue, but somewhat grayish (sunset and
down excluded).
    Incidentally, birds are animals and therefore there are blue
animals. There are also blue lizzards, but in mammals it is quite rare
indeed. In mandrils, a large african baboon, there is blue pigmentation
on the face.  One should also mention that the blue in bird feathers is
not produced by pigments, its origin is the same as the blue of the
sky, namely Rayleigh scattering by tiny air bubbles in the feathers.
--
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