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Where Does a Bird’s Magnetic Sense Reside?


In the new Illustra documentary Flight: The Genius of Birds, arctic terns are showcased as masters of global navigation, making a phenomenal 70,000-km circuit each year from pole to pole. Scientists believe that, like the Monarch butterflies seen in Illustra’s previous film Metamorphosis, birds combine several cues to navigate: the angle of the sun, the stars, and the Earth’s magnetic field. How and where do birds sense magnetism? Science Now says,

In the days before GPS, we needed both a compass and a map to navigate. Migrating birds are no different. Studies have suggested that the animals rely on an internal map and compass to traverse large distances, though just where these senses reside is unclear. Now, scientists say they have the strongest evidence yet that map sense is associated with the beak. (Emphasis added.)

The beak contains areas rich in iron, the article explains. Prior research had been inconclusive about a beak-to-brain connection from those iron particles to the brain. Recently, a German team cut the nerve between those regions and the brain in half of a group of Eurasian reed warblers, then moved the whole population from their normal take-off grounds in Russia to the east, 1000 km away. Here’s what happened as a result:

The warblers that had their beak-to-brain connection cut flew northeast, as if they had departed from near Kaliningrad -- they had lost their "map sense" and could no longer determine their location. Those with the nerve intact, on the other hand, quickly oriented themselves and turned northwest, toward their breeding grounds, the team reports this week in PLOS ONE. This meant that the beak-to-brain system, which, according to the earlier tests, had no impact on the "compass sense," did matter for the "map sense" of the birds -- if the link was damaged, the birds simply did not know they had been displaced.

But the experiments are still inconclusive, so far, to explain all the observations. More iron clumps have been found in the birds’ inner ear, Current Biology reports:

This organelle is found in hair cells in a wide variety of avian species, but not in rodents or in humans. This structure may function as (1) a store of excess iron, (2) a stabilizer of stereocilia, or (3) a mediator of magnetic detection. Given the specific subcellular location, elemental composition, and evolutionary conservation, we propose that this structure is an integral component of the sensory apparatus in birds.

By “evolutionary conservation,” the authors meant no evolution was found. The trait is common to all birds. Further, no precursor in reptiles was identified, nor any ancestry for the organelle. It appears evolutionary theory is useless for understanding bird migration.

Magnetic sensing is suspected in a wide variety of unrelated animals, from sea turtles to birds, from insects to fish, and even in some bacteria. Some mammals, like bats, appear capable of magnetic orientation. Even herds of cows have been found aligning north and south for unknown reasons. Birds and butterflies, as the Illustra films show, really excel at magnetic navigation. Even after many years of study, though, the location of the magnetic sense is still a mystery.

Like Science Now said, an animal needs a map and a compass; having just one can still get you lost, as in the case of the warbler experiments. It appears these two required elements lie in separate regions of the body and brain of navigating birds.

As you marvel at the navigational skills of arctic terns in Flight: The Genius of Birds, realize that scientists are struggling to understand how the birds do it. An intelligent design approach to solving the puzzle will expect a multi-part, complex system working together for function, taking the attitude Paul Nelson says at the beginning of the film: “If something works, it’s not happening by accident.” ID can also stimulate engineers to mimic this ability in their own designs.

Image credit: Wikipedia.