In the moment after their plane was hit, there was no time to think, let alone radio their position. The four Royal Air Force pilots ditched their broken bomber and dropped into the North Sea, near Britain. It was February 23, 1942 — and it should have been their last day on Earth.
Floundering in the frigid water, the pilots released their last hope: a tiny, bedraggled carrier pigeon named Winkie. She'd been inside a container the whole flight and was covered in oil from the crash. It wasn't clear that she would survive the 120-mile flight back home, or know how to get there.
But a few hours later, Winkie showed up at the home of her owner, who notified British authorities in time to launch a rescue mission. Without her, the four men might never have been found in the vast ocean.
So how did she do it?
"We think they are using quantum mechanics to navigate," said Daniel Kattnig, a researcher in the chemistry department at Oxford University. Kattnig works in a lab that studies radical pairs — a phenomenon in which atoms acquire extra electrons that are "entangled" with one another, each affecting the other's motion even though they're separated by space. It's a field of science that's difficult to understand under the best of circumstances; imagine trying to figure out it out with a bird brain.
But according to an increasingly popular theory, birds and other animals use a radical pair-based compass to "see" the Earth's magnetic field, allowing them to undertake great migrations and daring rescues without getting lost. It's still unproven, but Kattnig and his colleagues just verified a key component: In a study in the New Journal of Physics on Thursday, they report that the timing of these subatomic interactions makes them a good candidate to explain avian navigation.
"There are still many steps before we can say this for certain," Kattnig said. But this is one step along the way.
People have been trying to understand how animals know where they're going for more than 100 years. In a letter to Nature Magazine in 1873, Charles Darwin speculated that a sense of "dead reckoning" might allow everything from migratory birds to traveling tribes in Siberia to keep a course in rugged or unfamiliar terrain. Since then, scientists have proposed animal compasses based on sense of smell, memorized landmarks, the direction of the sun, polarization of light and even the positions of the stars. (It's been suggested that dung beetles plot a path back to their burrows by following the Milky Way.)
In the early 1960s, a German graduate student named Wolfgang Wiltschko set out to prove that birds navigated based on radio signals from the stars. During his experiments, he locked robins in a steel room with a Hemholtz coil — a device that produces a uniform magnetic field — and realized that the birds were reorienting themselves in response to it. He'd accidentally demonstrated that magnetism, not radio waves, was at the heart of animal navigation.
Those results sent scientists on a frenzied search for animals' magneto-receptors. They discovered iron particles in the beaks of pigeons and hens, magnetite in the noses of trout, and other magnetic molecules in the ear hairs of birds.
Subsequent research found that some of those iron molecules were in immune cells rather than sensory ones, shaking up the migration-by-magnetic-molecules theory. But animal navigation scholars already had another possible mechanism: the radical pairs that Kattnig studies.
When the idea was first proposed by biophysicist Klaus Schulten, then of the Max Planck Institute, a reviewer at the journal Science wrote back to him, "A less bold scientist would have designed this piece of work for the wastepaper basket," he recalled in a history published by the University of Illinois at Urbana-Champaign.
"But there are lots of behavioral experiments that show this is actually a good fit," Kattnig said.
It's thought that light-sensitive proteins called cryptochromes — which have been found in the retinas of birds, butterflies, fruit flies, frogs and humans, among others — are at the center of the mystery. When light strikes the proteins, it creates radical pairs that begin to spin in synchrony; they're entangled.
The chemical reaction lasts only for a few microseconds, but Kattnig's research shows that it's long enough for the Earth's magnetic field to modulate the quality and direction of the electrons' spin. He also found that the radical pairs become more sensitive to the magnetic field as they "relax" — that is, as they transition back to equilibrium — if you take into account outside factors like ambient temperature.
This suggests to Kattnig and his colleagues that sensors in the bird's eyes survey the spin state of various radical pairs and then signal the results to the brain, allowing birds to more or less "see" the Earth's magnetic field as they fly through it.
There's still years of work to be done, Kattnig acknowledged. "We need to locate the spot where the cryptochromes are responsive to magnetism," he said. "And then we need to find the interaction partners — the cascade of signals which is then following up and giving rise to the visual impression."
"Lots of things are unknown," he concluded.
To us, maybe. The birds are finding their way just fine.