I don't blog about research as a rule, but when I get the chance to use "Magnetic Cows" as a title, how can I resist? Here's a fun paper in PNAS by Burda et al with the delicious title of "Extremely low-frequency electromagnetic fields disrupt magnetic alignment of ruminants". They determine empirically that cows can sense magnetic fields (no Open Access - bad PNAS - but one of the authors have a copy on their homepage (click the English title)).
Apparently cows and deer tend to stand in a north-south direction more often than not. One hypothesis has been that they have a sense for the earth magnetic field; another is that they use visual cues, such as tracking the sun, to do so. It's difficult to do controlled experiments with such large animals, so the authors instead studied what happens with herds around high-voltage power lines, as the Earth magnetic field is disrupted near them. They looked at satellite and flyover images of herds near power lines, and studied deer behavior directly.
And true enough, cows standing near power lines would orient completely randomly, while cows well away from the power lines would tend to orient north-south. More detailed analysis of intermediate distances shows that no other cue (orientation of the power lines themselves, of instance) was responsible and that the compass mechanism, however it works, must be based on the magnetic field intensity rather than field inclination.
The big question in my mind is why cows and deer want to keep north-south? The authors speculate briefly that it may be a way to keep the herd synchronized, or perhaps a navigation strategy much like when we turn ourselves to match the map in our hands. Right now, though, nobody actually knows the real reason. So that cow over there may be a slow, dull meat and milk producer in your eyes, but she's still able to sense something you can't, for reasons she knows and you don't.