Birds begin fall exodus

Birds are starting the countdown for the big annual shift from northern regions to their winter homes. Some, like purple martins, will head deep into South America, while bald eagles will stack up at open water as close as Wabasha, Minn.

Baltimore orioles are gearing up to go, as are warblers and white-throated and other sparrows. Marshes are emptying of red-winged blackbirds and grasslands of sedge wrens and dickcissels. Indigo buntings, rose-breasted grosbeaks and brown thrashers won't be seen again until spring.

The bulk of fall migration occurs between the end of August and the middle of October. Many assume birds leave to escape the coming cold, but the truth is that migrants depart because their sources of food disappear as summer winds down.

Very few insects are available for kingbirds in winter, there's little fruit for catbirds, and fish and frogs are locked beneath the ice, safe from herons and osprey. In order to keep eating, many bird species must move southward. (Some seed-eating species -- like cardinals and finches -- and those capable of searching out hidden insects -- like woodpeckers and chickadees -- stay behind.)

As the sun sets earlier each day, birds' systems release hormones that lead them to gorge on foods that pack on fat. Other hormonal messages start the molt of a fresh new set of flight feathers for long journeys.

Solo flights

Birds become restless as their takeoff time approaches. However, the timing is not entirely hard-wired: Adult birds rely on experience to choose a departure night that has beneficial conditions. Each bird departs and flies on its own schedule, although it will share the skies with large aggregations of other birds.

Juveniles are able to find their way because they have a map embedded in their brains, telling them how many days to fly southward, then how long to tack westward, and where to end up. In later years, these birds will factor in cues such as the sun and star positions, magnetic fields and landmarks such as mountain ranges and rivers to help them navigate.

Flying golf balls

Male ruby-throated hummingbirds are among the early migrators, beginning their journeys in mid-August. These tiny birds may fly thousands of miles to reach Central America, so they gorge until they're as fat as golf balls before departing.

Swallows that skimmed over parks and golf courses all summer long left weeks ago. They'll continually replenish their tanks by feeding on insects during their daytime flights. Eagles and hawks fly in daylight, as well, in order to take advantage of rising columns of warm air -- thermals -- for the soaring flight that lessens their workload. (Not all migrants travel at night, although most songbirds do. Night flight allows for more time to feed during the day, and night's stiller and cooler air saves energy.)

Fall migration can be a fairly languid affair, with many birds traveling short distances before dropping down to feed and rest, and stopping for several days between travel days. They're not in a mad rush to get where they're going, unlike spring migration, when birds race to be first in line for the best territories.

Migration is a dangerous business, with many hazards, including storms, loss of stopover habitat, pesticides and oil spills. But birds intrepidly pit their tiny bodies against this gauntlet twice a year in an event almost unrivaled in the natural world for its immensity and sheer glory.

Val Cunningham, a St. Paul nature writer, bird surveyor and field trip leader, can be reached at valwrites@comcast.net.

NIGHT CALLS

A major advance in studying bird migration occurred in the 1990s, when researchers began recording birds' calls during night flight, different from daytime calls, and then using computers to sort them by species.

For the first time, this gave a good picture of what kinds and how many birds were passing overhead in the dark.

To watch migrating hawks, eagles and other raptors, no spot is better than Hawk Ridge in Duluth. Find out more at www.hawkridge.org/visit/visit.html.

To see migrating songbirds, visit Wood Lake Nature Center in Richfield, Carpenter Nature Center in Hastings or Lilydale Regional Park in St. Paul. Fort Snelling State Park is also a good spot, but check first to see if any roadways are closed.