COME wintertime thousands of garden warblers, pied flycatchers, and bobolinksall tiny songbirdswill cross the equator heading south for sunnier climes. It is an epic trip. For guidance they will rely on the position of the sun and stars, as well as smells and other landmarks. They may also use the Earths magnetic field, thanks to a sense known as magnetoreception. Theories about it have long attracted quacks. Franz Anton Mesmer, a German doctor working in the late 1700s, argued that living things contain magnetic fluids, which, when out of balance, lead to disease. His idea of animal magnetism was debunked and similar ones viewed with scepticism. But magnetoreception has drawn more serious attention in the past half-century. A pioneering study in 1972 demonstrated that European robins respond to magnetic cues. The list of animals with a magnetic sense has since grown to include species in every vertebrate category, as well as certain insects and crustaceans. Some may use it simply to orient, such as blind mole rats. Otherssalmon, spiny lobsters, thrush nightingalesmay use it for migration and homing, alongside other sensory cues. How do they do it?