Scientific American had a podcast a few days ago that said cows tend to either face north or south. The people who came to this conclusion studied Google Earth photos of cows (!) and noticed how they tend to line up. This is the first suggestion of a magnetic sense in large mammals.
Some other posts about Scientific American podcasts:
What makes the Mentos-Diet Coke trick work
Telescope on the dark side of the moon
Comet dust looks like asteroid dust
Water and epistemology
Applying page rank idea to biology
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John Venier 09.03.08 at 11:35
Maybe it is related to the sun rather than magnetic fields. After all, if you catalogued the orientations of fixed solar panels you’d see a similar alignment. And it seems much more likely to me that cows would respond to sunlight.