Who hasn’t thrilled to the sight of V-formations of migrating geese as they wing their way north to their nesting grounds, or heard them squawking overhead in autumn as they head back south to spend the winter in milder climates? This amazing, predictable movement occurs twice each year, involving millions of birds worldwide, traveling hundreds and even thousands of miles. Truly, “migration is the most spectacular of bird movements.”1
But for centuries this phenomenon was shrouded in mystery. Why do birds migrate and when do they travel? Where are they headed and how do they know where to go?
Until recently, erroneous theories abounded. Aristotle believed that certain species hid in holes in the ground and in trees, where they became featherless until the following spring. Carolus Linnaeus, who gave us the binomial classification system for all living creatures, believed that swallows wintered underwater.2
But clues began to appear two centuries before Linnaeus. In 1555 a Frenchman named Pierre Belon wrote from firsthand experience, “As swallows cannot spend the winter in Europe both because of the great cold and because they would not find food, they go to Africa, Egypt and Arabia, where, since winter resembles our summer, they have no lack of nourishment.”3
Migration is inextricably linked to the seasons, but how do birds know the seasons are changing? What causes the change in seasons?
That simple observation answered the question “Why?” for virtually all migrating species. They need warmth and food to sustain life and raise young, but these are not available in all seasons. Migration is inextricably linked to the seasons, but how do birds know the seasons are changing? What causes the change in seasons?
Because the earth is tilted 23.5° on its axis as it orbits the sun, the Northern Hemisphere is tilted toward the sun in summer and away from the sun in winter. The reverse is true in the Southern Hemisphere. Lengthening days provide more energy for warmth, plant growth, insect activity, and snow melt—freeing up vast areas for nesting and raising young.
Experiments show that this increase in day length triggers several amazing inbuilt changes in birds—physiological changes in their bodies (feather molt, increase in body weight, and development of the sexual organs) and migratory restlessness (a strong urge to depart, flight in one particular direction, and nocturnal flight in some species that normally fly during the day).4 The temperature also influences the timing and progress of spring migration. In one study, birds paused at their stopover sites when nighttime temperatures dipped below 70°F (21°C), and they started moving again when temperatures rose.5
The spring migration in the Northern Hemisphere is protracted, lasting three to four months. Birds choose one of several major migratory flyways—some fly over the North American continent through Mexico to the Southwestern U.S. and beyond, some fly across or around the waters of the Gulf of Mexico, and others fly across the Caribbean islands to Florida and points north.
Turkey vultures are the first migrants to return to their nesting grounds, around the first week of February. Next are the American robins, around the first week of March if the snow has melted. Then come the ducks and geese, the warblers, orioles, buntings, thrushes, tanagers, and others, reaching a crescendo during the first week of May.6
In many parts of North America, Europe, and Asia, the progress of perching birds (warblers, orioles, etc.) coincides with the “green line,” the first appearance of leaves in the deciduous trees, along with the first appearance of larvae of several insect species that eat leaves. This is no accident. The insects provide the birds with a protein-rich diet, while the birds save the trees from defoliation.7
Biologists have discovered that all long-distance migrants seem to have an amazing internal clock which triggers their movements north—an endogenous (internal) rhythm within the bird.
Canada geese and some other migratory birds winter in the southern U.S., which has a changing temperate climate. The coming of spring is obvious because it brings lengthened days and warmer temperatures. But what about migrants that winter on the equator or in the Southern Hemisphere? These birds are met with equal or shortening lengths of daylight and colder temperatures. How do they know when to travel north? Biologists have discovered that all long-distance migrants seem to have an amazing internal clock which triggers their movements north—an endogenous (internal) rhythm within the bird. 8
Another amazing feat of migratory birds is how they find their way to their specific nesting grounds. Researchers transported a Manx Shearwater from its nest burrow on Skokholm (a small Welsh island) to Boston, 3,107 miles (5,000 km) away across the Atlantic; the bird flew home in 12 days.9 How is this journey possible? Scientists have discovered that birds have an accurate sense of direction because of three compasses that track the sun, stars, and magnetic field.10
Their sun-compass (inner biological clock) enables them to compensate for the daily movement of the sun across the sky, so they can accurately determine compass direction at all times of the day. Experiments have also shown that the birds use the sunrise or sunset angles to fix their direction of migration for the day.11
Nocturnally migrating birds rely on their star-compass to keep them on course. Experiments in a planetarium in the 1970s showed that night-migrating birds use the Pole Star to determine their northward direction, regardless of the time of night.12
A bird’s magnetic compass is facilitated by minute crystals of magnetite, which were discovered in the heads and neck muscles of several North American bird species in 1980. These crystals enable the bird to orient itself in relation to the earth’s magnetic field during migration.13 This helps them stay on track even when it’s cloudy or raining.
Until recently we did not know where many bird species wintered, but modern technology has begun unraveling some of these mysteries. Spectacled eiders, which are sea ducks with striking patterns, literally disappeared when they left Alaska and northeastern Siberia. Where did they go for the winter? In 1994, tiny transmitters were implanted in the bodies of fourteen ducks, beaming their coordinates to orbiting satellites. When a signal indicated that the birds were in the middle of the Bering Sea, scientists exclaimed, “Unbelievable!” Then two biologists flew to the indicated location in March 1995, astounded to find 155,000 eiders safely resting in the small openings of an ice pack, more than 100 miles from the nearest land.14
Arctic terns were long known to have the longest migration of any animal in the world, and we knew their starting and ending points: in the summer they nested mainly north of the Arctic Circle, and they wintered in Antarctic waters (a 24,000-mile round trip). But we didn’t know their route of migration. In 2007 a team of scientists traveled to Sand Island off the northeast coast of Greenland, where 1,000 pairs of terns nested every year.15 They attached geo-locators weighing just 1.4 grams to the legs of 50 terns. The team returned in July 2008 and were able to retrieve 10 of the geo-locators. They revealed that the birds flew south over the Atlantic to waters on the equator. There the flock split, half continuing down the eastern Atlantic near Africa, while the others followed the curve of South America. The birds reunited in November in the Weddell Sea, where they fed on the abundant small fish during the Antarctic summer.
Within days in early April, all the birds began migrating north. This time they traced an S-shaped route together in the Atlantic to take advantage of favorable winds. They arrived back on Sand Island in early June, once again to lay eggs and raise their young. Each Arctic tern may repeat this incredible migration for its lifespan of 30-plus years, flying 2.4 million kilometers (equivalent to traveling to the moon and back three times). Truly an amazing migration!
The fingerprints of the Creator God are all over these amazing migrations, which scientists do not yet—nor ever will fully—understand. The Prophet Jeremiah eloquently stated, “
Even the stork in the heaven knows her appointed times; and the turtledove, the swift, and the swallow observe the time of their coming” (Jeremiah 8:7).