Physicists often simplify or idealize phenomena to make them more amenable to an initial mathematical treatment. We jokingly refer to this as considering a "spherical cow". Sometimes one can understand even very subtle phenomena using this technique. However, there are always important effects that one needs the full, non-symmetric nature of the situation to understand. Here, from The Telegraph, is an example of experimental data illustrating just this point (emphasis mine)!
Dr Sabine Begall and colleagues from the University of Duisburg-Essen looked at thousands of images of cattle on Google Earth in Britain, Ireland, India and the USA. They also studied 3,000 deer in the Czech Republic. The deer tended to face north when resting or grazing. Although, in many cases, the images were not clear enough to determine which way the cattle were facing they were aligned on a north/south axis. The scientists concluded that they were behaving in the same way as the deer. Huge variations in the wind direction and sunlight in the areas where the beasts were found meant that the scientists were able to rule out those factors as being responsible for the direction they were facing. "We conclude that the magnetic field is the only common and most likely factor responsible for the observed alignment," the scientists wrote in an article published in the Proceedings of the National Academy of Sciences journal.
All joking aside, I found this fascinating. It is hard to see why this feature would be useful to cows these days, but if you accept the evil theory of evolution, things become a lot clearer.
Their innate ability to find north is believed to be a relic from the days when their wild ancestors needed an accurate sense of direction to migrate across the plains of Africa, Asia and Europe.