Why do some birds spend the winter in Minnesota?

The gray catbirds departed in September, taking their feline-like calls and fondness for berries with them to Central America.

The Cirque de Soleil-like acrobatic barn swallows disappeared in late September from fields and other open spaces to hunt insects in Mexico and points farther south.

And the nectar-loving ruby-throated hummingbirds began taking their leave in mid-August, when males were on the move, followed by females and youngsters in September.

That's all part of fall migration, one of the twice-a-year waves of billions of birds between their winter homes and summer breeding grounds. Fall brings the return flight, clearing out forests, meadows, lakes and marshes of all but the year-round residents, taking about 75% of birds that nest in our region away. By now, the birds remaining in Minnesota are the smaller population that can make a living with the diminished food resources available in winter. Relatively few birds live in the same place year-round.

It seems counterintuitive, but experts tell us that the main reason birds migrate is not to escape the cold — many migratory birds could probably survive even Minnesota's winters. But they can't survive without food, and as their food sources — fruit and insects, for many songbirds — disappear, they do, too.

When we think about migration, we tend to focus on long-distance migrants such as Baltimore orioles, which head to Costa Rica, Panama and farther south each fall. Something like 350 species of North American birds fly thousands of miles twice each year.

They have to be very hardy birds, because migration is a severe test of their physical and mental abilities. Among the hazards: a lack of reliable food supplies along the long route, inclement weather, exposure to predators and loss of habitat. The options are stark: fly thousands of miles and dodge all the dangers, or stay put and hope to find enough food to stave off the cold. Surprisingly, studies are showing that birds that migrate have higher rates of survival than the birds that remain behind.

Researchers have many more tools for studying migration these days and are turning up some surprises. Birds that don't fly as far, called mid-distance migrants, spend the winter a few hundred miles away, and these include blue jays and goldfinches, birds we see all winter. But we don't see the same birds in winter as in summer, since many have moved southward, and birds from farther north move down to fill their niche.

This occurs in my own backyard, where I put out a big handful of peanuts-in-the-shell for blue jays each morning. All summer long, a crew of four jays shows up right after I put out the peanuts, busily snatching them and flying off to hide them around the neighborhood. But then, at the end of August or early September, the peanuts begin to pile up in the feeder, with no takers. This indicates the changing of the guard, as "my" jays are on their way to Iowa or Missouri, while jays from the north are moving in but don't know yet about the peanuts.

Birds live arduous lives, whether they migrate or stay put as the seasons change. Only about 30% of young songbirds survive their first year, and the hazards of migration are a major factor.

Give a thought to the billions of feathered beings who rise into the skies twice each year, passing overhead, often unseen, to make their living at a new address.

St. Paul resident Val Cunningham, who volunteers with the St. Paul Audubon Society and writes about nature for local, regional and national newspapers and magazines, can be reached at valwrites@comcast.net.