The Homing Instinct: Meaning and Mystery in Animal Migration. Bernd Heinrich
real estate and the defensive capabilities of the resident males — to gauge the possibility for future takeovers.
Loons nest on the ground along shorelines, if islands are not available. Shorelines, if low, are risky nesting places, not only because of potential flooding in early spring, but also because they are within easy reach of raccoons, skunks, and other predators. Most birds test what is a “good home” by direct experience — success in raising a brood there. Or, like the loons, they assess the experience of others: whether or not chicks have been raised there. So, if the territory does yield young, a floater, who apparently finds this out through scouting, may risk a fight in an attempted takeover to remove the defending male (if he achieves takeover, he automatically gets to keep the female who remains on her home). But the floater can risk an attack only when he is about four or five years old. When he is in prime physical condition and has much to gain — namely, a potential lifetime of home ownership — it makes sense to risk the battle. The older territory owner might then fight to the death, presumably because he has much to lose, and almost no chance to gain another home.
Loons may seem extreme in the lengths to which they go to secure a home, as do other birds that risk the hazards of migrating thousands of kilometers. Yet, in the movie I watched with rapt attention on board ship on the way to America at age ten, people on ponies shot arrows at others on a wagon train. All were emotionally charged, because each was fighting for something sacred, and therefore each was willing to risk his life, for defending or wanting a home.
We have learned much about thousands of animal species that twice annually risk their lives to migrate to an exact pinpoint, such as an oceanic island in the case of the albatross or a pond in the vastness of a continent in the loon’s. They open a window with a broad view onto our unending quest into the mysterious minds of animals, and in the process they illuminate our own. How does one tie the vastness that includes other animals, and so much that affects us personally and socially, together into a story?
Writing this book reminded me of when, after riding more or less unconscious in the slipstream of history for over fifty years, I started putting down stones chosen from a vast array of differently shaped fieldstones to build a house foundation. I couldn’t chop them to size or knock the edges off so that they would fit into the inevitable empty spaces to make neat connections. Nor did I want to shape them, like bricks, to make a tidy but artificial structure. The stones found in nature, like facts, are endlessly numerous, wild, and complex. As the famous British geneticist and evolutionary biologist J.B.S. Haldane quipped, “My own suspicion is that the universe is not only queerer than we suppose, but queerer than we can suppose.” I hope to give here a view of some of the “stones” of homing, and their origins, and how they apply in real life.
Homing is central to many aspects of our and other animals’ lives. To understand the meaning of home, like any other phenomenon, it helps to step back and see from another’s world. Animals give more than just clues to the why and the how of homing. They show what is possible, what has been tested, and what has worked over millions of years of evolution.
In this book I cannot hope to provide an in-depth treatment of any issue. My viewpoint is wide, and admittedly personal. Thousands of scientific references are possible for any one topic, and I am not an expert on any topic. Those references I cite are in no way meant to be the specific last word on the topic. They tend to be those I am most familiar with. I apologize for all of the amazing stories and all the details that I have left out. I have tried to speculate freely, and I hope that this will open discussion, not close it.
A skull I have lying on my desk is as big around as a somewhat flattened coffee mug, but it comes to a point at the front and has two large eye sockets. The bone on the top of the head is sculpted in ridges and furrows and looks like weathered stone. Seen from the back, the skull has a backward-opening hole on each side. The holes anchored the animal’s powerful jaw muscles. Between them is a much smaller hole. I am looking through this hole (foramen magnum) for the brain cavity, but this large head contains scarcely any brain at all, just an extension of the dorsal nerve cord that runs up from the neck.
It’s the skull of a snapping turtle, a female who met her end as she was crossing the road to dig a hole in the gravel to lay her clutch of eggs. As in previous years, she had come from her bog a mere hundred meters away.
The local snapping turtle is no great wanderer. But two sea turtles — the green turtle, Chelonia mydas, and the leatherback, Dermochelys coriacea — are renowned for migrations spanning entire oceans. The leatherback as an adult feeds in northern cold waters but nests on tropical beaches. It can weigh up to a ton, and although the skull of one that I had the opportunity to examine was as big as a basketball, the cavity holding its brain was, like the snapping turtle’s, only a slight expansion at the end of the vertebral column. It could barely have held a walnut. The green’s could have held two hazelnuts. Their skulls hardly differ from that of any turtle, whether of a species alive now or one that lived 215 million years ago, at a time at least three times more distant than that of the last dinosaurs.
The minute dimensions of some animals’ brains are as astounding as the homing capacity of some of their owners. Like albatrosses, sea turtles of various species lay their eggs in colonies with others of their kind on specific ocean beaches. After the young hatch from the eggs buried in the sand, they head for the water and spend years at sea. They may travel thousands of kilometers, and then, a decade or two later when they are ready to lay their eggs, they return to their birthplace. They mate in the water nearby, and the females then come ashore to dig their nest holes in the sand and to drop in their eggs. How are they able to find their old home after years of wandering in the vastness of the oceans, when we, if taken blindfolded to and then released in unfamiliar woods, would, despite our highly sophisticated massive brains, be as likely to head off in a wrong as a right direction? To get around in unknown territory most of us need a map with which to find at least one known fixed feature that we can both see on the ground and locate on that map, and a compass.
What knowledge and what kind of urges does it take for some birds to fly nonstop for nearly ten thousand kilometers, spending all day and night on the wing, until their body weight halves as they not only burn up all of their body’s food stores but even sacrifice muscle, digestive tract, and other entrails — almost everything except their brains?
If feeling fails you, vain will be your course.
— Johann Wolfgang von Goethe, Faust
MILLIE AND ROY ARE A PAIR OF SANDHILL CRANES THAT STAY for most of the year in Texas or Mexico but travel north in April and have for at least fifteen years nested and raised their one or two offspring, known as colts, in a small bog in the Goldstream Valley near Fairbanks, Alaska. Their home is adjacent to the home of my friends George Happ and his wife, Christy Yuncker. George was an insect physiologist and chairman of the Zoology Department at the University of Vermont where I was hired in 1980, and he later moved to the University of Alaska and the land of the Iditarod, where the two built their home in the wild land near Fairbanks. They invited me to visit them and “their” cranes, and I was eager to do it.
The thousands of square kilometers of central Alaska’s permafrost-covered taiga consist of stunted blue-green spruce and white birch, with a groundcover of green-yellow moss and twiggy Labrador tea