A river of birds migrates over the Mississippi

It's just plain hard to sleep on autumn nights if I start thinking about one of the most awe-inspiring events in the natural world, the annual passage of billions of birds on their way south.

Most songbirds travel at night, when it's cooler and fewer predators are on the wing. Hundreds, even thousands of these birds pass in the darkness over my house and neighborhood each evening as they begin their journeys to winter homes. Mixed flocks might include rose-breasted grosbeaks, gray catbirds, tiny warblers and the whole family of thrushes.

Something like 204 of the bird species that breed in Minnesota migrate away each fall. Some, like the blackbird family, travel in daylight. But on some nights, there truly is a virtual river of birds overhead, although their numbers ebb and flow over the migration season. Streams of birds heading south converge from the north, west and east. And this river of birds then stays on course on the journey's first leg by following a key landmark, the Mississippi River, a major migratory flyway.

Many of us assume that birds travel on autopilot, following directional instructions embedded in their genes, but we now know that birds use a variety of signals to create a migration map.

Navigation over long distances is a complex business: Birds assess light patterns from the setting sun and a reading of the local magnetic field as they start out each evening. Along the way, they look up at the stars and down to landmarks on the ground, such as rivers and mountains, to stay on course. Over time, after a number of migrations, they develop a sense for the best route, storing up vital information on good replenishing sites and where to stop to rest. (Young birds do rely for their first trip on instructions hard-wired into their brains, then modify it as they gain experience.)

Some, like bluebirds, may travel only a few hundred miles. But long-distance champs like purple martins, scarlet tanagers and blackpoll warblers may cover thousands of miles before their final touch-down. Some of our summer birds spend the winter among such rain forest exotics as trogons, motmots and manakins.

Migration is the most dangerous and energetically expensive thing birds do in their lives. They fly through storms (even hurricanes), face predators, dodge communication towers and tall buildings, and encounter sites freshly sprayed with pesticides and drastically altered landscapes. Another major hazard is loss of habitat, places to stop and refuel during their flights.

Migration and other hazards add up to a high mortality rate for birds. In fact, fewer than 30 percent of young songbirds survive their first year of life.

If migration is so dangerous, why did birds evolve this way? Why don't they just spend the entire year in Louisiana, or Mexico?

It's really all about food. The birds we call Neotropical migrants used to be year-round residents of the tropics. But after northern glaciers receded some 10,000 years ago, songbirds began moving north in the spring to take advantage of our wide-open spaces and insect "bloom," the huge insect hatches typical of our springs and summers. Since nearly all songbirds raise their broods on this high-protein food source, it's to their advantage to travel hundreds or thousands of miles for abundant food. They race northward, claim nesting territories, stuff their nestlings with insects, watch them fledge, then head south again.

A pair of songbirds feeds their nestlings up to 9,000 caterpillars in the space of just two weeks, before they fledge. These birds and others take advantage of the fact that insects are protein-rich, more so, it turns out, than beef.

So the daunting risks of migration are offset by many advantages, especially the availability of food, with the result that migratory birds produce more offspring, on average, than sedentary tropical songbirds.