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A melodramatic picture of a pigeonEveryone knows that migrating birds are capable of incredible feats of navigation; for example, the Bar-tailed Godwit manages to navigate across the Pacific during its non-stop 11,000km flight from Alaska to New Zealand. Some birds use visual or olfactory cues to navigate, but many birds are able to sense the Earth’s magnetic field, an ability which is supposed to be underpinned by a group of iron-rich cells in the upper beak. However, a study just published in Nature has uncovered the true identity of these cells and shown that they’re probably not involved in sensing magnetism, re-opening the question of how birds can navigate across thousands of kilometers.

Successful navigation needs both a compass to determine which direction you’re going in and a map to tell you where you are. Based on a study with robins, we know that birds use light-dependent chemical reactions in their eyes as a compass; they know which direction they’re flying in because these reactions are sensitive to the orientation of the Earth’s magnetic field. Birds might taking advantage of another property of the magnetic field in order to know where they are. The intensity of the Earth’s magnetic field gets weaker from the Equator to the poles; if birds could sense the strength of the magnetic field, they could use that as the basis of a magnetic map. It’s been thought that birds accomplish this using a group of iron-rich cells in their upper beak; earlier studies have claimed that these cells were magneto-sensitive neurons because the iron in them is in the form of a magnetic mineral called magnetite.

To investigate this, a team of researchers led by Dr. David Keays stained the beaks of over 200 pigeons with a chemical which reacts with iron and used MRI scans to create a precise map of the iron-rich cells, which are supposed to be restricted to six bands in the beak. That’s not what the researchers found, though. Instead, they saw a huge amount of variation in the number and distribution of cells. “We found a startling diversity in the location and number of iron-rich cells in the pigeon beak. Some birds had just 200, while others had over 100,000,” said Dr. Keays, “it just didn’t make sense that these cells were the bird’s sat-nav system”.

The iron-rich cells also didn’t react with several antibodies used to label neurons, suggesting that they might not be neurons after all. Under the microscope, they were seen to have tentacle-like extensions and sometimes appeared to be engulfing other cells, which made the team think they might be a type of white blood cell called a macrophage. The researchers were able to confirm this by using a chemical stain which labels macrophages and seeing that 95-98% of the iron-rich cells were stained. Since macrophages should be found throughout the animal and not just in the beak, they also checked samples from other parts of the birds, including the back, abdomen and wing; all of the samples contained iron-rich cells which were indistinguishable from those in the beak. Finally, they used an electron microscopy technique to determine that the iron in these cells was probably not in magnetite, but in minerals like ferrihydrite and goethite, which aren’t associated with biological sensitivity to magnetic fields. Altogether, this is pretty conclusive evidence that these iron-rich cells are part of the birds’ immune system and not magneto-sensitive neurons.

This leaves open the question of how the birds actually navigate. “We have no idea how big the puzzle is or what the picture looks like, but today we’ve been able to remove those pieces that just didn’t fit” said Dr. Keays. I think it’s pretty amazing that we still don’t actually know how birds navigate when they’re migrating. We’ve made incredible progress in learning about the world around us, but studies like this remind us that we’ve just barely scratched the surface. It’s really invigorating to realize how much we still have to discover and understand, even about seemingly mundane things that happen in our own backyard.

I also think this paper is an excellent example of the way science works — or the way it should work. Sometimes, we’re wrong. The strength of scientific practice lies in recognizing that fact and incorporating it into the process of deciding what to believe. Any idea is welcome to compete on the playing field; how well it will be accepted depends on whether it can explain the things we know and reliably predict the things we don’t know. Science isn’t a set of beliefs, but a system for deciding what to believe. Unlike some other belief systems, science can cope with creating beliefs that are wrong — in fact, science thrives on discovering and correcting these mistakes. I think it’s important to emphasize the idea of science as a method and a process instead of as some kind of received knowledge, since that very process is what distinguishes science and is the reason for its enduring appeal and power.

Ref:
Treiber, C., Salzer, M., Riegler, J., Edelman, N., Sugar, C., Breuss, M., Pichler, P., Cadiou, H., Saunders, M., Lythgoe, M., Shaw, J., & Keays, D. (2012). Clusters of iron-rich cells in the upper beak of pigeons are macrophages not magnetosensitive neurons Nature, 484 (7394), 367-370 DOI: 10.1038/nature11046

The earlier study about the light-dependent magnetic compass:
Zapka, M., Heyers, D., Hein, C., Engels, S., Schneider, N., Hans, J., Weiler, S., Dreyer, D., Kishkinev, D., Wild, J., & Mouritsen, H. (2009). Visual but not trigeminal mediation of magnetic compass information in a migratory bird Nature, 461 (7268), 1274-1277 DOI: 10.1038/nature08528

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