Don’t Go Towards the Light, Unless You’re a Bird
It could be said that birds come equipped with their own compass. They generally stick to the same migratory patterns year after year and always seem to know which direction to go. It wasn’t until recently though that researchers discovered their “inner compass” does not work by magnetic sensing cells in the beak, but instead they use a visual center in the brain and light-sensing cells in the eye to find which way is the cardinal north.
Henrik Mouritsen of Germany’s University of Oldenburg—and co-author of the study—says “this is basically the sixth sense of biology, but no one knows how it works. The magnetic sense is by far the least understood sense in the natural world.”
Up until the new findings, researchers found themselves pitted against each other over how birds detected magnetic fields. One group of scientists claimed that the light-sensing cells in a bird’s eyes can sense the magnetic field and send information to a light-processing part of the brain known as “cluster N”. Those that disagreed with the theory believed it was a simple matter of iron-based receptors found in cells of the upper beak that could sense the magnetic field and send information to the brain via nerves. Needless to say, the light-sided argument wins this round.
So, how did they find the location of these secret, inner compasses? Well, Mouritsen and his colleagues caught 3 dozen European robins and checked that each one could orient themselves with natural and artificial magnetic fields. Then, they performed surgery on the birds, either cutting the trigeminal nerve or damaging the brain cells in cluster N. This way, they were able to tell which theory was true; the light sensory or the iron receptors.
One of the things Mouritsen mentions is that the birds’ eyes contain special proteins called “cryptochromes”. These cryptochromes may control how the birds sense magnetic fields through light. When light hits the proteins it produces a pair of free radicals—highly reactive molecules with unpaired electrons. These electrons have a spin property that could be sensitive to Earth’s magnetic field; and if so, then signals from the free radicals transfer to nerve cells in cluster N, thus pointing the bird in the right direction.
Things aren’t so simple for all migratory bird species though. If a bird has portions of their cluster N that are damaged, they may no longer be able to sense or orient themselves to magnetic fields. A study conducted on such birds revealed that they could not pick up on the Earth’s natural magnetic field or artificial ones created by scientists. As for the magnets in the beak theory? The trigeminal nerve—that which connects the beak to the brain—was severed in a number of birds, and they still were able to orient themselves fine. This goes to prove that the iron-like receptors in the beak has nothing to do with a bird’s ability to migrate properly.
By understanding more about how birds navigate and sense the world around them, conservation efforts may improve for their sake. Birds have a habit of returning to their seasonal nesting grounds. I’m sure that those of you who live in the north have seen the birds fly south for the winter, and come spring they’re right back to their home, sweet home. Unfortunately, some of their homes also fall victim to habitat destruction, poaching and so forth. By figuring out how they navigate, conservations may be able to keep the birds in safer areas for longer periods of time.
Now, if scientists can figure out why some birds also tend to migrate backwards, I, for one, would be really impressed.
By Heidi Marshall