3 billion, or 29 percent, over the past 50 years.

BIRDS THAT GO THE DISTANCE

• Birds that fly the longest distances have the longest and most sharply pointed wings—the most aerodynamic design for lengthy flights.

• Ruby-throated hummingbirds, just 31 ⁄2 inches long, can fly 1,245 miles without a break. Many take a straight line across the Gulf of Mexico to Central America.

• The blackpoll warbler, a 51 ⁄2-inch songbird whose favorite breeding territory is the boreal forest covering much of Canada and Alaska, flies 2,300 miles to South America and the Caribbean.

• Migrating birds can fly high as well as far. Songbirds have been tracked at altitudes of 2,000 feet. Geese and raptors may go up to over 30,000 feet.

• For all of their endurance, migrating birds aren't above hitchhiking. Many have stopped on ships while crossing water, sometimes in huge numbers if the weather is bad.

• When the wind is con trary on long flights, the result is often a fallout— unusually large numbers of birds stopping in one place at the same time. Although fallouts are a thrill for bird watchers, the birds have a rough time, fighting first the weather and then avian crowds vying for food.

• One last, little-known fact: Whatever the time of year, there are always some birds on the move for migration. 

Photo: Brooks Elliott/Getty Images

For absolute distance, arctic terns are the migratory champions, nesting in the Arctic every summer and then flying to Antarctic latitudes for another summer. Depending on the directness of its routes, one arctic tern may make the equivalent of three round trips to the Moon over the course of its 30-year lifetime.

Katherine Swarts lives in Houston, Texas, near the Central Flyway, a "superhighway" for migratory birds.