The science and history of why some birds migrate

Fall songbird migration is over; most of our breeding species have returned to their tropical homelands. They'll be back in the spring for another nesting season.

Seasonal climate prompts migration, writes author Roger F. Pasquier in his new book "Birds in Winter" (Princeton University Press).

Earth spins on a tilted axis, 23.5 degrees from vertical. This varies the amount of sunlight we receive, creating our seasons.

If Earth's axis was vertical, Pasquier writes, a semitropical climate would extend as far north as 50 degrees latitude. Our border with Canada is at 49 degrees.

Migration for many of our current species extends back 15 million years or longer. Fossil remains show that some birds had the anatomy required for long-distance flight at least 100 million years ago.

Most of the birds we will watch this winter are non-migrants. Or they are species that have adapted to weather quite the opposite to that found in their former homes, the South and Central American tropics.

Cardinals are one of the adaptive species. They have an interesting Minnesota story. More in a moment.

The Palearctic is a biogeographic region that includes Europe, Asia north of the Himalayas, and Africa north of the Sahara. On our side of the globe we have the Nearctic, Greenland and North America north of tropical Mexico. Biogeographic refers to the plant and animal life within those regions.

Those Palearctic species crossed what we know as the Bering Strait during one of the periods when cold climate captured water as glaciers, creating a land bridge from Siberia to North America, according to Pasquier.

Species that spend the winter with us made that crossing — woodpeckers, corvids (jays, crows, ravens), chickadees, nuthatches and finches. Included are short-distance migrants like bluebirds and thrushes, species wintering in our southern states.

Staying north through the winter offers advantages, Pasquier says. A bird wintering in its breeding territory increases its breeding potential by saving time. "The risk," he says, "is that they may not survive through the cold winter."

Our migrants from the tropics could have remained year-round in their warm and stable climate. When they decided to migrate for the breeding season they made that trade.

If you come north from the tropics to nest you find less competition for food and breeding territory. But the time spent in migration is time lost for breeding.

Cardinals have made themselves at home in both worlds. They are found in Central America as well as at your feeder. Cardinals are one of the tropical species that expanded its breeding range north.

The growth of bird feeding helped cardinals survive winters, as did a warming climate.

Cardinals initially settled in southeastern states. Bob Janssen, who knows Minnesota bird history very well, told me that the first known record for a cardinal here is Oct. 23, 1875, in Minneapolis.

There are scattered records from southern Minnesota until 1913. At that point, "records really started to increase as far north as Hennepin County," he said. "By 1924 cardinals had become a common permanent resident in southeast Minnesota."

Manitoba birders reported cardinals in the early 1930s. These sightings were dismissed as caged bird escapees. Records since that time indicate a true range expansion, according to Manitoba breeding bird survey records.

"Climate models predict higher temperatures for Manitoba over the next several decades," survey records state, adding that this could alter cardinal distribution there.

It certainly has made a difference here, slowly but surely.