How Migratory Birds Find Their Way Home

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Ever wondered how a bird finds its way home after breeding season?

It all comes down to a sense called magnetoreception, which is the ability to "see" the Earth's magnetic field. Birds, bats, and bees all possess this skill – as do dolphinssharks, and the supervillain Magneto. (Sadly, other humans do not, though this professor says we just might.)  


Scientists used to think the mechanism responsible for this "sixth sense" took place in iron-rich cells in the bird's beak, but the latest research seems to point to a protein in the bird's eye, specifically a light-sensitive cryptochrome protein called Cry4. Two recent studies, one looking at European robins in Current Biology and a second looking at zebra finches in the Journal of the Royal Society Interface, have found further evidence to support this theory.

In the first, a team from the University of Southern Denmark and the University of Oldenburg, Germany, used computer microscopy to compare the activity of the four known cryptochromes found in robins' eyes.

While the first three seemed to have no involvement in magnetoreception whatsoever, the fourth – Cry4 – varied in concentration depending on the point in the bird's migratory cycle, suggesting this particular protein plays an important role in migration. During the migratory season, levels were high, whereas in non-migratory seasons, the amount of Cry4 produced was noticeably lower. 

In the zebra finch study, researchers monitored levels of Cry1, Cry2, and Cry4 in the brain, muscles, and retinas of 39 birds to see how they changed over a circadian day. Cry4 remained constant, as the researchers suspected it might. In contrast, Cry1 and Cry2 rose and fell throughout the day. 


"We hypothesized that retinal cryptochromes involved in magnetoreception should be expressed at a constant level over the circadian day, because birds use a light-dependent magnetic compass for orientation not only during migration, but also for spatial orientation tasks in their daily life," the study authors explained. "Cryptochromes serving in circadian tasks, on the other hand, are expected to be expressed in a rhythmic (circadian) pattern.”

Thus, they conclude, Cry4 is most likely the mechanism behind magnetoreception, behaving like an internal “magnetic compass”.

The protein’s quantum interactions could help birds sense this field, Atticus Pinzon-Rodriguez from the University of Lund, Sweden, who was involved in the study, told Science News

While the science strongly indicates Cry4 is the biological basis for magnetoreception, the researchers are quick to point out it is not yet a certainty.


“We have quite a lot of evidence, but [Cry4] is not proven,” Henrik Mouritsen, an animal navigation expert at the Institute of Biology and Environmental Sciences in Oldenburg, Germany, who was involved in the robin study told Science News

[H/T: Science News]


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