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FUNCTIONAL TEST. “Light-dependent magnetic compass information is transmitted from the retina through the optic nerve to the visual thalamus and from there to Cluster N in the forebrain via the thalamofugal visual pathway. If Cluster N is destroyed, European robins can no longer use their magnetic compass.” https://lnkd.in/eUbuD-a View in LinkedIn

RADICAL PAIRS. “The reference direction provided by the Earth’s magnetic field is detected in the birds’ eyes, where cryptochrome proteins are the most likely light-dependent magnetic sensory molecules. Light absorption is thought to generate long-lived flavin-tryptophan radical pairs within cryptochromes in the retina, the reaction yields of which are determined by the orientation of the molecule with respect to the geomagnetic field vector.” https://lnkd.in/eUbuD-a View in LinkedIn

BIRD STRUCTURE. “These findings have since sparked a search for putative magnetoreceptors in the inner ear of birds. Using correlative light and electron microscopy, a previously undescribed iron-rich organelle has been discovered in the cuticular plate of avian hair cells. The organelle ranges from 300 to 600 nm in size and is comprised of ferritin-like granules. The structure is conspicuous given that almost 100% of hair cells contain only a single iron-rich organelle and seems to be limited to birds.” http://rsif.royalsocietypublishing.org/content/12/110/20150499.full View in LinkedIn

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NO CANDIDATE STRUCTURE. “Elaborate iron-mineral-based structures thought to be magnetoreceptors have also been reported in the upper beak of birds. However, recent findings suggest that these structures are macrophages involved in iron homeostasis. Thus, at present, no convincingly documented, iron-mineral-based, magnetoreceptive candidate structures are known from birds.” https://lnkd.in/eUbuD-a View in LinkedIn

MAGNETIC CHAINS. “Single domain magnetite particles similar to those found in the magnetosome of magnetotactic bacteria have been observed from the olfactory tissues in a number of fish, including species of tuna, salmon and trout. Early studies used SQUID to describe the magnetic properties of frozen samples and revealed the presence of 40–100 nm-sized SD magnetite particles arranged in chains.” https://lnkd.in/eAwwGJ4 View in LinkedIn

UNSOLVED. “Many studies have documented the presence of magnetite or some other kind of iron mineral crystals in almost any animal...However, the mere existence of iron mineral crystals or even magnetite does not represent significant evidence by itself that such structures have any relevance to magnetoreception.” https://lnkd.in/eUbuD-a View in LinkedIn

BACKGROUND NOISE. “Thus, any magnetic-field-based input to a reasonably precise map would require a very accurate magnetic sensory system and a very accurate sense of gravity. However, even if birds have such a system, daily, partly stochastic, natural variations in the geomagnetic field on the order of 30–100nT in more or less random directions mean that it is very difficult to imagine how a magnetic-field-based map sense could have a precision less than 10–30km (during magnetic storms generated primarily by the Sun, the geomagnetic field variability can reach 1000 nT.” https://lnkd.in/eUbuD-a View in LinkedIn

SUBTLE SIGNAL. “The problem for a bird wanting to use a magnetic map is that the average change in magnetic field intensity is only c. 3 nT/km on the North-South axis: The geomagnetic field changes c. 30,000 nT from one of the magnetic poles to the magnetic equator, which are c. 10,000km apart. Likewise, magnetic inclination changes only c. 0.009°/km along the North-South axis (a 90° change over 10,000km).” https://lnkd.in/eUbuD-a View in LinkedIn

MULTIFACTORIAL. “One thing is for sure; the natural map sense of birds is multifactorial. It relies on input from olfaction and vision and possibly also from magnetic sensing and maybe even from hearing. Pigeons with opaque lenses that prevented them from detecting any local visual landmarks can return to within c. 5 km of their loft.” https://lnkd.in/eUbuD-a View in LinkedIn