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From: dmark@sunybcs.uucp (David Mark)
Newsgroups: sci.bio,sci.misc,rec.birds
Subject: Re: animals and Earth's magnetic reversal
Message-ID: <9294@sunybcs.UUCP>
Date: 14 Mar 88 23:23:34 GMT
References: <7387@ihlpa.ATT.COM>
Sender: nobody@sunybcs.UUCP
Reply-To: dmark@sunybcs.UUCP (David Mark)
Organization: SUNY/Buffalo Geography
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In article <7387@ihlpa.ATT.COM> cutler@ihlpa.ATT.COM (Herber) writes:
>
>Scientists, or those who profess to know such things, indicate that
>the earth has reversed it's magnetic polarity many time is the past.
> Has anyone given thought on the resulting effect this might have on
>animals, migratory birds in particular, that use this field for 
>whatever purposes?  Will Northern hemisphere birds migrate north
>in the winter?  Are there any indications of specie disappearance during
>these times?

It turns out that migration cage studies in Europe have shown that
(at least some) birds use the *slope-direction* of the Earth's
magnetic field, rather than the polarity, to orient during migration.
The lines of magnetic force plunge downward in the direction toward
each pole.  The experiment went something like this:

1) capture a southbound migrating warbler in the autumn and put it in
   a cage; the cage has a floor which slopes toward the cage center
   (funnel shaped), and an ink-pad in the center.  The bird leaves
   footprints on the funnel which can be analyzed statistically to
   determine the direction that the bird is trying to fly.

2) surround the cage with a coil in which a magnetic field, considerably
   stronger than the Earth's, can be generated.

3) - when the coil tilts downward toward true north, and the polarity
   agrees with the Earth's natural direction, the bird continues to
   try to fly south;
   - when the polarity is reversed but the tilt is not changed, the
   bird keeps flying south (toward the 'north' polarity of the
   artificial field);
   - however, when the polarity is kept the same as the Earth's but the
   tilt is tipped southward, the birds turn around and head northward
   (toward the direction that the lines of magnetic force head upward
   from the horizontal).

If reversals of the Earth's magnetic field took place in a short time (less 
than a year), there would be a very strong selective pressure in favor of this 
system and against against a polarity-based navigation scheme.  I don't recall
how many species have been tested in this way.  I could look up the reference
if anyone is interested.  It was described in a small but good monograph
on bird navigation; my recollection is that the author's name was
Schmidt-Koenig.

David Mark
dmark@joey.cs.buffalo.edu
geodmm@ubvms.BITNET