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1
Introduction
The winter frost has arrived early this year in Europe and there is a penetrating chill in the evening air. Buried deep within a young robin's mind, a once vague sense of purpose and resolve grows stronger.
The bird has spent the past few weeks devouring far more than her normal intake of insects, spiders, worms and berries and is now almost double the weight that she was when her brood flew the nest back in August. This extra bulk is mostly fat reserves, which she will require as fuel for the arduous journey upon which she is about to embark.
This will be her first migration away from the spruce forest in central Sweden where she has lived for the duration of her short life and where she reared her young chicks just a few months ago. Luckily for her, the previous winter was not too harsh, for a year ago she was not yet fully grown and therefore not strong enough to undertake such a long journey. But now, with her parental responsibilities discharged until next spring, she has only herself to think about, and she is ready to escape the coming winter by heading south to seek a warmer climate.
It is a couple of hours after sunset. Rather than settle for the night, she hops in the gathering gloom to the tip of a branch near the base of the huge tree that she has made her home since the spring. She gives herself a quick shake, much like a marathon runner loosening up her muscles before a race. Her orange breast glistens in the moonlight. The painstaking effort and care she invested in building her nest—just a few feet away, partially hidden against the moss-covered bark of the tree trunk—is now a dim memory.
She is not the only bird preparing to depart, for other robins—both male and female—have also decided that this is the right night to begin their long migration south. In the trees all around her she hears loud, shrill singing that drowns out the usual sounds of other nocturnal woodland creatures. It is as though the birds feel compelled to announce their departure, sending out a message to the other forest inhabitants that they should think twice before contemplating invading the birds' territory and empty nests while they are gone. For these robins most certainly plan to be back in the spring.
With a quick tilt of her head this way and that to make sure the coast is clear, she takes off into the evening sky. The nights have been lengthening with winter's advance and she will have a good ten hours or so of flying ahead of her before she can rest again.
She sets off on a course bearing of 195° (15° to the west of due south). Over the coming days she will carry on flying in, more or less, this same direction, covering two hundred miles on a good day. She has no idea what to expect along the journey, nor any sense of how long it will take. The terrain around her spruce wood is a familiar one, but after a few miles she is flying over an alien moonlit landscape of lakes, valleys and towns.
Somewhere near the Mediterranean she will arrive at her destination; although she is not heading for any specific location, when she does arrive at a favorable spot she will stop, memorizing the local landmarks so that she can return there in the coming years. If she has the strength, she may even fly all the way across to the North African coast. But this is her first migration, and her only priority now is to escape the biting cold of the approaching Nordic winter.
She seems oblivious to the surrounding robins that are all flying in roughly the same direction, some of which will have made the journey many times before. Her night vision is superb, but she is not looking for any landmarks—as we might were we making such a journey—nor is she tracking the pattern of the stars in the clear night sky by consulting her internal celestial map, as many other nocturnal migrating birds do. Instead, she has a rather remarkable skill and several million years of evolution to thank for her capacity to make what will become an annual autumn migration, a trip of some two thousand miles.
Migration is, of course, commonplace in the animal kingdom. Every winter, for instance, salmon spawn in the rivers and lakes of northern Europe, leaving young fry that, after hatching, follow the course of their river out to sea and into the North Atlantic, where they grow and mature; three years later, these young salmon return to breed in the same rivers and lakes where they spawned. New World monarch butterflies migrate thousands of miles southward across the entire United States in the autumn. They, or their descendants (as they will breed en route), then return north to the same trees in which they pupated in the spring. Green turtles that hatch on the shores of Ascension Island in the South Atlantic swim across thousands of miles of ocean before returning, every three years, to breed on the exact same eggshell-littered beach from which they emerged. The list goes on: many species of birds, whales, caribou, spiny lobsters, frogs, salamanders and even bees are all capable of undertaking journeys that would challenge the greatest human explorers.
How animals manage to find their way around the globe has been a mystery for centuries. We now know that they employ a variety of methods: some use solar navigation during the day and celestial navigation at night; some memorize landmarks; others can even smell their way around the planet. But the most mysterious navigational sense of all is the one possessed by the European robin: the ability to detect the direction and strength of the earth's magnetic field, known as magnetoreception. And while we now know of a number of other creatures that possess this ability, it is the way the European robin (Erithacus rubecula) finds her way across the globe that is of greatest interest to our story.
The mechanism that enables our robin to know how far to fly, and in which direction, is encoded in the DNA she inherited from her parents. This ability is a sophisticated and unusual one—a sixth sense that she uses to plot her course. For, like many other birds, and indeed insects and marine creatures, she has the ability to sense the earth's weak magnetic field and to draw directional information from it by way of an inbuilt navigational sense, which in her case requires a novel type of chemical compass.
Magnetoreception is an enigma. The problem is that the earth's magnetic field is very weak—between 30 and 70 microtesla at the surface: sufficient to deflect a finely balanced and almost frictionless compass needle, but only about a hundredth the force of a typical fridge magnet. This presents a puzzle: for the earth's magnetic field to be detected by an animal it must somehow influence a chemical reaction somewhere in the animal's body—this is, after all, how all living creatures, ourselves included, sense any external signal. But the amount of energy supplied by the interaction of the earth's magnetic field with the molecules within living cells is less than a billionth of the energy needed to break or make a chemical bond. How, then, can that magnetic field be perceptible to the robin?
Mysteries, however small, are fascinating because there's always the possibility that their solution may lead to a fundamental shift in our understanding of the world. Copernicus's ponderings in the sixteenth century on a relatively minor problem concerning the geometry of the Ptolemaic geocentric model of the solar system, for instance, led him to shift the center of gravity of the entire universe away from humankind. Darwin's obsession with the geographical distribution of animal species and the mystery of why isolated island species of finches and mockingbirds tend to be so specialized led him to propose his theory of evolution. And German physicist Max Planck's solution to the mystery of blackbody radiation, concerning the way warm objects emit heat, led him to suggest that energy came in discrete lumps called "quanta," leading to the birth of quantum theory in the year 1900. So, could the solution to the mystery of how birds find their way around the globe lead to a revolution in biology? The answer, bizarre as it may seem, is: yes.
But mysteries such as this are also a haunt of pseudoscientists and mystics; as the Oxford chemist Peter Atkins stated in 1976, "the study of magnetic field effects on chemical reactions has long been a romping ground for charlatans."1 Indeed, all manner of exotic explanations, from telepathy and ancient ley lines (invisible pathways connecting various archaeological or geographical sites that are supposedly endowed with spiritual energy) to the concept of "morphic resonance" invented by the controversial parapsychologist Rupert Sheldrake, have at some point been proposed as mechanisms used by migratory birds to guide them along their routes. Atkins's reservations in the 1970s were thus understandable, reflecting a skepticism prevalent among most scientists working at that time toward any suggestion that animals might be able to sense the earth's magnetic field. There just did not seem to be any molecular mechanism that would allow an animal to do so—at least, none within the realms of conventional biochemistry.
