How do birds know when to migrate?

How do birds know when it's time to migrate?

It's a good question because when it comes to migration, birds are complicated mysteries dressed in feathers.

Darrell in Brooklyn Center asked me about migration recently. He had juncos at his bird feeders in late November, he told me, asking how they knew when to leave their breeding territories. That's Canada for juncos.

We can ask the same question about any migrant bird species.

In the 17th century, scientist Charles Morton declared that birds migrate to the moon and back each year. He estimated an outbound trip of 60 days.

Philosopher Aristotle suggested that redstarts transformed into European robin species for the winter, one species leaving as the other arrived. Made sense to him.

For centuries many people believed that summer birds, such as swallows and martins, buried themselves in wet mud and slept out the winter underwater.

Even now, "The mechanisms initiating migratory behavior vary and are not always completely understood," according to the Cornell Lab of Ornithology's "Handbook of Bird Biology."

Changes in day length, lower temperatures, changes in food supplies and genetic predisposition combine to trigger migration, the book says.

The change in natural sunlight triggers hormones that make birds restless, gather in flocks and eat more food to fatten up for the long journey ahead, according to Cornell.

For centuries, people with caged birds noticed that migratory species would go through a period of restlessness each spring and fall. They repeatedly fluttered toward one side of their cage, the direction of the migration urge they were feeling.

German behavioral scientists gave this behavior the name zugunruhe, which means migratory restlessness. It comes from the German words "zug" (meaning to move) and "unruhe" (restless anxiety).

An article in Scientific American magazine suggested that the magnetic compass of migratory birds "relies on quantum effects in short-lived molecular fragments known as radical pairs formed photochemically in the eyes."

In this way, it explained, birds can perceive Earth's magnetic field lines, using that information for navigation. Earth's magnetic fields, by the way, are 10 to 100 times weaker than a fridge magnet, according to the article.

Peter J. Hore, a chemist at the University of Oxford, wrote in Scientific American that "We are still a long way from proving how migratory birds perceive Earth's magnetic field lines."

Perhaps in a month I'll notice longer days. The sun began its trip north on Dec. 21, the solstice, so on Dec. 22 and days beyond we had more daylight.

Most of us wouldn't notice the one-minute gain in daylight that occurred on Dec. 21. Birds are aware of subtle physical changes that we mostly miss.

The cycle with which we might be most familiar is known as the circadian (circa = periodicity, -dian = cycle). It's a cycle found in all living things, daily to annual cycles for you, your dog, the plant in your window, aphids on the plant, the birds in your yard.

Juncos, by the way, are part of the New World sparrow family. In Minnesota, we see the dark-eyed junco, that single species, according to Robert Janssen's book "Birds of Minnesota."

They are among the most numerous of North American forest birds, uncommon in Minnesota as breeders. They are apt to be seen as winter visitors.

We also see plumage variations known as Oregon and pink-sided and slate-colored versions of the dark-eyed junco. These usually are designated as subspecies, a taxonomic category that ranks below species, usually indicating a fairly permanent geographically isolated race.

Junco systematics (the branch of biology that deals with classification and nomenclature), however, are debated by various bird authorities. Anywhere from three to 12 species and/or subspecies are recognized by someone.

For most of us, however, to paraphrase Gertrude Stein, a junco is a junco is a junco.

Lifelong birder Jim Williams can be reached at woodduck38@gmail.com.