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If you think you've got a long commute, think about all the birds heading south this fall. As this ScienCentral News video explains, scientists say some of the smallest ones use night-vision to find their way.
A Bird's Eye View
One of nature's great phenomena is how tiny songbirds can make their way over thousands of miles each fall to their winter feeding grounds. Scientists have known for years that they travel by night to avoid predators, navigating by the stars and the Earth's invisible magnetic field. Yet how these birds "see" the Earth's magnetic field — a protective field that shields Earth from radiation, and is the basis for the magnetic north and south poles, but which people can't sense at all — has remained a mystery.
Now researchers based in the United States and Europe have found a brain region in night-migrating songbirds that they think can "process" information from the Earth's magnetic field and turn it into an internal compass they can see. The brain region is called "Cluster N" — "N" for night-vision because the researchers believe the birds' ability to sense the Earth's magnetic field and transform it into a navigation tool is dependent on their ability to see at night.
The earth's magnetic fields. image: NASA
"What we discovered was that this brain area wasn't exclusively used for sensing magnetic fields, but instead it's being used to perhaps see at night," says Duke University neurobiologist Erich Jarvis.
"This area is only active in the night-migratory birds at night… and it's never active in the non-migratory birds, not during the day, nor during the night," says Mouritsen who published the finding with Jarvis in the journal Proceeding of the National Academy of Sciences.
Mouritsen and Jarvis used a combination of behavioral observations and genetic tests to identify Cluster N. In each of their labs, they set up Plexiglass cages, each with a compass fixed on top. They monitored individual birds from each group with video cameras to see how each species behaved overall. During the day all the birds just hopped around aimlessly. But at night, both sets of migrating birds pointed themselves in a southerly direction and flapped their wings vigorously in an attempt to fly south, while the non-migrating birds made no attempt to fly away.
When the researchers compared the brains of the migrating birds to the non-migrating birds they saw a bright white area in the forebrain of the migrating birds. They only saw this highlighted area in the migrating birds at night and not during the day, so they think something about the dim light of the night sky activates Cluster N.
image: Henrik Mouritsen
"It just pops out at you… there's just no way you can ignore it," says Jarvis who used a technique he developed called behavioral molecular mapping to see what was happening in the birds' brains. "When an animal performs a particular behavior the areas of the brain that are responsible for performing that behavior… induce the synthesis of certain genes in the brain. You can actually use that induction of synthesis of genes to identify the area that was active in that particular behavior."
The researchers believe night vision triggers a cascade of events in these birds' brains that allows for the transformation of the Earth's magnetic field into what Mouritsen imagines looks like a compass or a radar target superimposed over their normal vision. While they aren't entirely certain yet, they think the first step in this transformation is the entry of dim, night-sky light into the eye. That agitates a molecule called chryptochrome, one of a few known to be affected by light and which is in abundance in the nerve cells of the eye that are active when a bird is trying to navigate. Mouritsen and Jarvis think these cells carry magnetic field information to Cluster N. They believe Cluster N then converts the information into a visual image.
While more studies will be needed to prove this hypothesis, the researchers have made the first step. When they blindfolded the migratory birds, and monitored them as they had previously, the birds made no attempt to fly south at night and Cluster N was never activated. "If the [night] light is away," says Mouritsen, "Then the first step in this processes is blocked."
The researchers say it is doubtful people have a similar ability or brain region. And if we do, they say it's a vestige of the past long since discarded on the path of evolution. But, University of California at Irvine bird migration specialist and biophysicist Thorsten Ritz says, the finding is still an "important step [in] identifying areas where further research can be done," and notes that magnetic fields are all around us and if we can figure out how birds see and process them, then we may be able to identify how magnetic fields affect us. And that Jarvis says, would be "one more discovery" made about humans based on a bird brain.
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