One of the theories for the incredible navigational skills of birds is that they can sense magnetic fields through the magnetite in their beaks — essentially giving them a built-in compass. It's one of the more pervasive explanations, but it looks like it might not be true, at least for pigeons.
New research published in Nature has shown that while there are indeed iron rich cells in a pigeon's beak, the previous research has entirely miscategorized their purpose. The researchers mapped the location of these cells in the beak, and found much greater variation in number and location than would work with being magnetic receptors.
Rather, these clusters are iron-rich macrophage cells, don't contain magnetite, and are most likely part of the pigeon's immune system, and used for iron homeostasis. It turns out these iron-laden cells are found throughout pigeons, and that means our interpretation of them as the primary method for these birds to sense magnetic fields is wrong. As the authors say:
Although we cannot exclude the possibility that a small number of sparsely distributed magnetoreceptors reside at an unknown location in the upper beak of pigeons, this study finds no evidence to support the existence of a subepidermal magnetic sense system that consists of iron-containing dendrites at six specific bilateral loci. This conclusion...has several important implications. First, it requires a re-evaluation of behavioural studies that have purported to impair the function of a magnetite-based receptor in the subepidermis of the upper beak and the conclusions that these studies reached. Second, it necessitates a re-assessment as to whether superparamagnetic magnetite has the necessary physical and magnetic properties to act as a magnetosensor in a living system. Third, our work reveals that the sensory cells that are responsible for trigeminally mediated magnetic sensation in birds remain undiscovered. These enigmatic cells may reside in the olfactory epithelium, a sensory structure that has been implicated in magnetoreception in the rainbow trout.
Which means we need to go back to the drawing board to figure out what birds are using for sensing magnetic fields, because it wasn't what we thought.
Image: Homing pigeons, once used to deliver the "pigeon post." Magnetic fields are believed to guide pigeons back to their home loft from unknown locations, sometimes more than 1,000 km away. Credit: Keays, et al.