interfered with the birds' orientation. The field would not have interfered with an iron-based compass. Further, birds are unable to detect a 180 degree reversal of the magnetic field, something they would straightforwardly detect with an iron-based compass. From 2007 onwards, Henrik Mouritsen attempted to replicate this experiment. Instead, he found that robins were unable to orient themselves in the wooden huts he used. Suspecting extremely weak radio-frequency interference from other electrical equipment on the campus, he tried shielding the huts with aluminium sheeting, which blocks electrical noise but not magnetic fields. When he earthed the sheeting, the robins oriented correctly; when the earthing was removed, the robins oriented at random. Finally, when the robins were tested in a hut far from electrical equipment, the birds oriented correctly. These effects imply a radical-pair compass, not an iron one. In 2016, Wiltschko and colleagues showed that cryptochrome can be activated in the dark, removing the objection that the blue light-activated mechanism would not work when birds were migrating at night. A different radical pair is formed by re-oxidation of fully-reduced FADH−. Experiments with European robins, using flickering light and a magnetic field switched off when the light was on, showed that the birds detected the field without light. The birds were unaffected by local anaesthesia of the upper beak, showing that in these test conditions orientation was not from iron-based receptors in the beak. In their view, cryptochrome and its radical pairs provide the only model that can explain the avian magnetic compass. A scheme with three radicals rather than two has been proposed as more resistant to spin relaxation and explaining the observed behaviour better. Iron-based The second proposed model for magnetoreception relies on clusters composed of iron, a natural mineral with strong magnetism, used by magnetotactic bacteria. Iron clusters have been observed in the upper beak of homing pigeons, and other taxa. Iron-based systems could form a magnetoreceptive basis for many species including turtles. Both the exact location and ultrastructure of birds' iron-containing magnetoreceptors remain unknown; they are believed to be in the upper beak, and to be connected to the brain by the trigeminal nerve. This system is in addition to the cryptochrome system in the retina of birds. Iron-based systems of unknown function might also exist in other vertebrates. Electromagnetic induction Another possible mechanism of magnetoreception in animals is electromagnetic induction in cartilaginous fish, namely sharks, stingrays,