When things go as they should in the natural world, following cycles set down ages ago, we tend to take them for granted, not giving much thought to the wonders around us.

Take the migration of birds: It may start subtly, without our noticing that the back-yard catbird is no longer calling or that orioles haven't been seen for some weeks. Robins, so visible in summer, either become an unusual sight or show up in flocks.

Swallows gather on utility lines, chattering noisily between great sweeping flight forays, and bluebird families fly overhead, singing their soft songs to stay in touch.

Fall migration clears forests, meadows, parks, lakes and marshes of all but the resident birds. Billions of them are already heading southward, or are getting ready to go.

About two-thirds of the birds that spend the warm seasons here will be gone by late fall, including thrushes, grosbeaks, most raptors, wading birds, shorebirds, hummingbirds and swifts. Some, like the chestnut-sided warbler and the rest of its clan, will travel long distances before setting down in the tropics for the winter. (We saw some of these handsome little birds in Cuba in March.)

Others, like robins, may travel only a few hundred miles, to warmer states where fruit is abundant.

Hawk Ridge Nature Reserve in Duluth is a great place to view fall hawk migration.

(Robins do their best to confuse us, because they are migratory, but some from farther north may settle in our area for the winter, making it seem as if they've stayed around all year. Goldfinches and blue jays are known to do this, too, with those heading south being replaced by more northerly birds, until the populations switch back in the spring.)

Credit is due

Birds may make it look easy, but migration is full of dangers and uncertainty. For example, fall migration peaks at prime hurricane season, when many birds need to cross the Gulf of Mexico. Think of tiny hummingbirds with feet gripped tightly on shrubs lashing in the wind as they wait to make the 500-mile crossing.

Or habitat disappears — think of vireos accustomed to stopping over for a few days in a Mexican woodland, only to find it replaced by vacation homes since their last trip. There are other dangers, too: Consider a flock of bobolinks, eager for a meal to replace calories lost in a long flight, settling down in a rice field, with no way to sense whether it has been poisoned with agrochemicals.

And still birds make the journey, twice a year, for as long as they live.

Not that these travelers have much choice, since the urge to migrate is built into their DNA. It's part of their survival repertoire, honed over thousands of years, to cope with the biggest threat of all. Many of us think birds leave to escape winter's cold, but the real driver is food scarcity in the cold months. Migration is the only sure way for the majority of birds to continue to enjoy an abundant supply of insects, or berries or fish, whatever makes up their daily dietary regime.

Night or day

Some birds may travel as little as 15 miles on a given day, pausing frequently to rest and forage, while others fly hundreds of miles.

Depending on the distance of their final destination, some birds travel for a few weeks, while others may be "on the road" for up to four months. They're in less of a hurry in the fall, since they aren't on the way to compete for the best breeding territories.

Birds that are fast fliers and those that rely on catching insects on their journey, such as swallows and kingbirds, fly during the daytime. So do ducks and loons, and raptors that soar on rising columns of sunlight-warmed air. Smaller birds fly during the night to avoid raptors and exploit the calmer air.

Birds lift off from their summer homes or staging areas where they gather to get ready. Guided by major geographical landmarks, such as the coasts, mountain ranges and major rivers, birds tend to move in great linear waves along aerial highways we call flyways.

Billions head out, but not all of them return. Fewer than 30 percent of young songbirds survive their first year, and the hazards of migration are a major reason.

Migration occurs year after year, but it's still worthy of our notice — and our awe. Billions of tiny beings fling themselves into the skies, without regard for hazard or danger, an ounce or two of sheer energy facing whatever comes their way.

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.

More on migration

Duluth's 'hot spot'

One of the best places to view fall migration is only 2 ½ hours north of the Twin Cities, at Duluth's Hawk Ridge. If wind and weather are right, thousands of hawks, eagles and songbirds may stream by the ridge, some even at eye level. Best viewing times are 10 a.m. to 2 p.m. each day. There are no guarantees, since some days only a few hawks or eagles may migrate past the ridge. But you may get lucky and visit on a day when the skies are filled with kettles, rising swirls of hundreds or thousands of raptors. Check out the ridge's website, www.hawkridge.org, for more information.

Migration close to home

Parks along the Mississippi River (Lilydale, Crosby Farm, Hidden Falls, Minnehaha Falls) can be worth a visit in early morning or early evening to observe migrants as they drop down after a long night of flying or head out on another leg of their journey.

You might also visit Wood Lake Nature Center in Richfield, Springbrook Nature Center in Fridley, Carpenter Nature Center in Hastings or Maplewood Nature Center in Maplewood to watch for migrants.

And don't forget your own back yard: So many birds are on the move that a few will almost surely drop down to check for food or a drink at some point. (Keep feeders filled and birdbaths fresh.)