Sloshing about some 1,800 miles below the surface of the Earth lies a 1,367 miles thick layer of liquified minerals composed largely of iron and nickel called the outer core. It’s liquified because it’s hot — 8,132 to 9,932 degrees F — it sloshes in response to the spinning of the Earth on its axis (1,029 mph at the equator).
Because of the lovely magic of physics, flowing metals in the outer core create electric currents which (again, because of the Earth’s rotation) create a magnetic field extending from the North to the South Pole.
Actually, the Magnetic North Pole is currently located about 223 miles southwest of the Geographic North Pole. I say currently because it’s exact location has been drifting NNW towards Siberia at about 25-35 miles per year since 1990. (The Magnetic South Pole is roughly 10, 630 miles southeast of the Geographic South Pole at present.)
The Earth’s magnetic field doesn’t spread uniformly through the atmosphere but in distinct lines of force. Place a child’s bar magnet on a piece of white paper and sprinkle it liberally with tiny iron filings. Tap the paper and watch the filings align themselves along the magnet’s lines of force. The earth’s magnetic field operates in a similar fashion.
It’s been known for some decades that many animals can sense these lines of magnetic force, but a 2021 study published in the prestigious scientific journal Nature provided our first clear insight into how birds might actually be able to “see” those lines for use as guides when migrating.
When a protein found in birds’ eyes called cryptochrome 4 (CRY4) was isolated from a European Robin in a laboratory experiment, it was found that blue light sensitized it to the Earths magnetic field. The process, which involves quantum mechanics, changes the form of the protein. (Google search “Science News quantum compass” for a concise summary of how this works.)
It’s hypothesized that the nervous system of migratory birds can, in an as yet unknown way, sense the change in CRY4 proteins in their eyes, enabling them to become aware of — and possibly even visualize — the Earth’s magnetic force lines. It is suggestive that CRY4 isolated from chickens, a nonmigratory bird, is much less sensitive to magnetic fields.
We’re learning that birds can use the Earth’s magnetic field in more ways than just telling north from south. Consider a famous study conducted by Richard Mewaldt in the winter of 1964.
After banding a group of White-crowned Sparrows in his backyard in California, he shipped half of them to Louisiana and the other half to Maryland. Next winter, birds from both areas had found their way to his backyard again, after having first flown to their traditional breeding grounds in the Yukon.
It’s now thought a likely explanation for such GPS-like behavior is that migrating birds can find their way across the face of the planet to specific locations because they had memorized the “magnetic signatures” of those areas when they were young. Here’s one example of how that might work:
The Earth’s magnetic lines of force are parallel to the surface of the Earth at the equator but increasingly angle downward as they near the north and south magnetic poles. The angle at a given location is called its magnetic inclination. A study published earlier this year presented strong evidence that juvenile reed warblers in Europe memorize the magnetic inclination in the area where they are raised.
After overwintering in Sub-Saharan Africa, they use this angle as a stop sign in deciding where to end their northward return migration. But birds have other orientation capabilities in their migration toolkit, too. It’s long been known that those that migrate during the day can use the position of the sun for determining direction, while nocturnal migrants refer to stars in the night sky.
Tricky things to do, though, since both methods require some sort of internal biological clock. To fly due north at noon, for example, you could fly with the sun directly behind you, but that plan wouldn’t work at 9 a.m. or 4 p.m. And rather than memorizing particular constellations, nocturnal migrants (like most of our songbirds) pay attention to the rotation of groups of stars around the stationary Pole Star as the night advances.
It turns out birds’ ability to synchronize their behavior with the time of day involves a complex set of interacting systems located in the SCN region of the brain, the pineal gland, and the retina.
There’s also evidence that various species make use of geographic landmarks, low frequency sounds like the rumble of ocean tides or wind topping a mountain range, the polarization of light at sunrise and sunset, and even olfactory cues in finding their way.
In the final essay of this three-part series on avian migration, we’ll consider how migration is thought to have evolved and how birds deal with the many hazards facing them during migration.
Ken Baker is a retired professor of biology and environmental studies. If you have a natural history topic you would like Dr. Baker to consider for an upcoming column, please email your idea to email@example.com.
This article originally appeared on Fremont News-Messenger: Do birds sense the Earth's lines magnetic force when migrating?