AS A LITTLE BOY with a passion for birdwatching, I joined the local Audubon Society. Each spring as we chased warblers, tanagers, and thrushes, the old timers would complain about the migration. The birds, they griped, were fewer, just a fraction of the numbers that used to pass by. I dismissed their laments. After all, these people were fifty, sixty, even seventy years old. Their diminished eyesight and hearing, or the frailties of memory, could explain any apparent declines in the number of songbirds. Now, nearly four decades later, I find myself complaining too. Either I am getting older or they were right. Or both.

In fact, all around us, the mass movements of birds, mammals, reptiles, fish, and insects that are so important to our environment, economy, and culture are under siege, victims of habitat destruction, overexploitation, and climate change. The flocks of songbirds that protect our forests by consuming defoliating insects have been fading since at least the early 1960s, according to data gathered by U.S. and Canadian birdwatchers. Since 1990, the U.S. Fish and Wildlife Service has placed over twenty-five populations of Northwestern salmon on the endangered list — victims of dams, irrigation projects, logging, and sprawl. In the Yellowstone region, nearly three-quarters of the migratory routes of pronghorn antelope have been severed due to farming, fences, oil and gas drilling, and residential development.

And yet scant public attention is being paid to the decline of the world’s great animal migrations — a singularly profound change in the natural world. Why? First, disappearing migrations do not equate to disappearing species — in most cases, the animals survive, but in diminished numbers. Accustomed to rescuing species at the brink of extinction, we tend to ignore those animals that are still relatively common, but becoming less so every year. Second, ecologists speak of the concept of “shifting baselines,” a process whereby each generation takes for granted that what it sees in nature is the norm. From one generation to the next, we lose track of how great the migrations used to be, so we fail to appreciate the magnitude of the loss.

History suggests that once a great migration is diminished, there is little prospect of re-creating it, usually because the breeding or wintering habitat, or the migratory route in between, has been destroyed or usurped by people. Thus, the time to save a migration is while the animals are still abundant. Governments and the public must be prodded to take action before a migratory species is actually endangered. And they must protect key habitats across jurisdictions and boundaries, including international borders. The latter requires a level of cooperation that is all too rare.

But it can be found. Canada and the United States signed a joint waterfowl management plan in 1986 that led to the protection of millions of acres of wetlands for ducks, geese, and swans. Decades ago, Kenya and Tanzania created protected areas on each side of their border to safeguard the Serengeti’s spectacular migration of wildebeest, zebras, and other mammals. Unfortunately, nothing comparable exists for bats, butterflies, dragonflies, pronghorn, and numerous other migratory animals.

If we strive to protect animal migrations in all their glory, we will protect much more: our forests and rivers will be healthier; important habitats from the Arctic to the Amazon will be secured; and future generations will be able to witness some of the greatest spectacles in nature.

If, on the other hand, we fail to protect the great migrations, our descendants are unlikely to be angry. They probably will have no idea what they are missing.