Pulses only appear to effect the orientation of adult migrating birds, not juveniles (Munro et al., 1997a,b; Holland & Helm, 2013), which suggests that the ferrimagnetic sense is involved in an experience-based mechanism possessed by adult but not juvenile birds. Because adults have true navigation, this suggests the ferrimagnetic sense is involved in the true navigation map. A recent study has also shown that migrating reed warblers returning to their breeding grounds, are unable

to correct for a displacement of 1000 km eastwards if the trigeminal nerve is cut, unlike intact and sham operated birds, who are able to do so (Kishkinev et al., 2013). This finding, on migratory birds, is in contrast Ferroptosis inhibitor clinical trial to the findings on homing pigeons, where no role for the trigeminal Torin 1 chemical structure nerve in navigation is supported. On this basis it is argued that migrating birds possess two magnetoreceptive pathways: a radical pair mechanism in the

eye, which is responsible for at least compass orientation, and a ferrimagnetic sense, which is implicated in the detection of magnetic intensity and is involved in the navigational map (Wiltschko & Wiltchko, 2007). However, caution is urged in accepting this interpretation without question. Adult but not juvenile migratory birds have been shown to respond to changes in intensity (Deutchlander et al., 2012) and adult but not juvenile migratory birds have been shown to be affected by a strong magnetic pulse (Munro et al., 1997a; Holland & Helm, 2013), but there is no direct causal link between the two. Similarly, the trigeminal nerve has been shown to be involved in detecting the magnetic field (Mora et al., 2004), the pulse effect no longer persists when this is anaesthetized, and migratory birds with trigeminal nerve section can no longer correct for displacement (Kishkinev et al., 2013), but there is no direct link between the pulse and magnetic intensity, or the trigeminal Neratinib nerve and magnetic intensity. Evidence for a ferrimagnetic sense that is responsible for detecting intensity as part of a true navigational map is thus based on several indirect links. We do not know for certain that the pulse affects

a receptor that detects intensity, only that it changes navigation behaviour and that the behaviour appears to be mediated by the trigeminal nerve. To be certain of that, we would need to know the nature and location of the magnetic receptor. Initially, iron-containing cells found in the upper beak of the homing pigeons and other birds were suggested as magnetoreceptors innervated via the trigeminal nerve, although no clear sensory receptor was identified (Beason & Nichols, 1984; Williams & Wild, 2001). A structure that has the potential to be a magnetic receptor has been described in the beak of homing pigeons (Fleissner et al., 2003), chickens Gallus domesticus, garden warblers Sylvia borin and robins Erithacus rubecula (Falkenberg et al., 2010).