Let’s consider the wonder of bird migration today, starting with a shorebird. During the summer, semipalmated sandpipers breed on the arctic tundra all across North America. As precocial birds, semipalmated sandpiper chicks hatch fully feathered and can find their own food soon after hatching. The parents provide some protection from predators, but the chicks largely feed themselves. In July, before the chicks can even fly, the adults depart for wintering areas near the mouth of the Amazon River in South America. Once the chicks learn to fly, they depart a few weeks later to an area where they have never been.

Semipalmated sandpipers are not the only birds whose young find their way unaided to wintering areas. Young-of-the year magnolia warblers find their way to Central America, Kirtland’s warblers to the Bahamas and Swainson’s hawks to Argentina. Geese and cranes are among the minority of birds that lead their young to wintering grounds. Most birds have their migration genetically encoded.

These internal maps are often amazingly precise. It is not unusual for a songbird over its lifetime to use the same breeding territory in Maine and spend the winter in the same wooded area in Mexico. Clearly, birds have marvelous navigational abilities. How do they do it?

The sun is an obvious guide for navigation. Captive European starlings become restless as the time for migration approaches. They attempt to fly from the cage in a particular direction, corresponding to their known migration direction. On cloudy days when the sun is obscured, the birds do not orient in the proper direction.

Most songbirds migrate at night so the sun is not a visible cue. However, some night-migrating songbirds use a sun compass by judging the proper direction of migration as the sun is setting and then remembering that direction when they begin their migration in the darkness.

Some migrant birds navigate using the stars. Captive songbirds that are ready to migrate orient in the proper direction on cloudless nights but move randomly when clouds block out the brighter stars. Some interesting experiments have been conducted in a planetarium. Birds inside the planetarium orient in particular directions based on the patterns of the stars. When the map of the stars is rotated so that north and south are reversed, the birds orient in the right direction as indicated by the stars, not the wrong direction as indicated by the earth’s magnetic field. Some of our local birds that are known to use the stars to guide their way are black-billed cuckoos, rose-breasted grosbeaks and bobolinks.

The earth’s magnetic field can be used to navigate by some migrating birds. Early experiments with pigeons showed that placing small magnets on the birds’ necks interfered with their ability to orient properly. More sophisticated experiments have been done recently in which birds are fitted with small helmets that reverse the direction of the magnetic field around the bird. The north pole of the experimental magnetic field around these birds is actually south. As expected, the birds orient in exactly the opposite direction.

Some birds that migrate during the day use visible landmarks for navigation. Hawks and eagles may follow mountain ridges. Some landbirds may follow the coastline.

Finally, some birds use their sense of smell to find their way. Homing pigeons with their nostrils plugged with cotton are not able to find their homes as well. Some seabirds like the Leach’s storm petrel locate their nest burrows by smell.

As many of our birds leave us until the spring, we can be thankful they can navigate so well. We know we will see many of them again next spring.

Herb Wilson teaches ornithology and other biology courses at Colby College. He welcomes reader questions and comments at:

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