Migration becomes treacherous business.

• Position: Your Life

The most far-reaching investigation to date on the journeys of migrating species shows drastic changes are underway in the U.S. and around the world. The migrations of many American species begin in September and continue through November and December.

Human-caused environmental alterations such as climate change, sprawling development, pollution, and overexploitation of wildlife and natural resources all pose major obstacles in the long-distance journeys undertaken by tens of thousands of animal species worldwide. Many are experiencing dramatic drops in population due to environmental changes in their breeding or wintering grounds or traditional resting areas along the way that have provided refuge for centuries, according to ecologist David S. Wilcove, author of No Way Home: The Decline of the World's Great Animal Migrations.

In Mid-Atlantic states such as New Jersey, Maryland, and Delaware, the red knot (a type of sandpiper) is experiencing sharp declines due to over-harvest of the horseshoe crab, whose eggs are the knot's main source of fuel during part of its 18,000-mile, round-trip journey. The move to develop oil and gas fields in the Rocky Mountain region threatens...