"Pick the right night and you will hear them. A September night, perhaps, when a northwest wind has swept the clouds from the sky and the stars are out in full force. As midnight approaches, find a quiet spot away from the rumbles and groans of urban life and listen carefully. Soon you will hear soft chirps and whistles drifting down from the sky. These are the calls of migrating songbirds. A thousand feet above you, extending for hundreds of miles in all directions, is a vast highway of little birds—millions of thrushes, warblers, flycatchers, tanagers, vireos and sparrows—heading south."

But maybe not for much longer—thanks to ongoing habitat destruction, the creation of obstacles such as fences or dams, overusing natural resources, and climate change.

So writes conservation biologist David Wilcove of Princeton University in his book No Way Home: The Decline of the World's Great Animal Migrations. Already, many of the world's largest migrations are gone: the million-bird flocks of passenger pigeons and hundreds of thousands of bison that roamed North America, the right whale that swam along the western coast of Europe, and the springbok gazelle that crossed South Africa.

The impact of these losses are unknown, but the ecosystem of the Pacific Northwest is surely suffering from the lack of nutrients caused by the decline of salmon runs from as much as 500 million pounds (226 million kilograms) of fish to as little as 26.5 million pounds (12 million kilograms) today—these rivers now receive a mere 6 to 7 percent of the nitrogen and phosphorus they once enjoyed. The situation is so dire, Wilcove and his colleague Martin Wikelski (who has attached tiny sensors to select dragonflies to understand how even insects migrate) write in a recent issue of PLoS Biology, that measures must be taken now before they disappear.

Scientific American.com
's David Biello spoke with Wilcove about this decline and what can be done to stop it.

Birds are still migrating, salmon are still spawning so what is the decline in migrations you are talking about?

The challenge is maintaining migration as a phenomenon of abundance. We need to protect these species before they become rare. The reasons are several: First, from a purely aesthetic point of view. What is so awe inspiring about migration is not seeing a handful of sandhill cranes on the Platte River, it's seeing half a million. It's not seeing a dozen wildebeests in the Serengeti, it's seeing a million. So much of the beauty stems from the sheer number of animals.

The second, perhaps more important, reason is the ecological role of the migrants. The ecosystem services they provide is tied to their abundance. There have been a number of studies that have shown that at least in temperate forests breeding songbirds reduce the level of insect damage to trees and crops. You can think of migratory birds as providing a pest control service. The value of that service depends on the number of birds.

Another example would be the salmon, which are really self-propelled bags of fertilizer, transporting nutrients from the ocean where they grow to adult size to the rivers and streams where they spawn, die and decompose. As you reduce the population of salmon, you reduce the transportation of nutrients.

In many respects, we have a stake in preserving the abundance of migrants, which is why their decline should be of concern even if the individual species are not in any real danger of going extinct.

What are the factors leading to this decline?
You can divide the threats to migrations into four: habitat destruction, human-created obstacles, overexploitation and climate change. Their severity depends on what species we're talking about and what part of the world we're in.

What is leading to the decline in songbirds?
I first started studying this issue when I was a graduate student over 25 years ago. People have been looking at this issue intensely for almost three decades now and there have been thousands of papers and hundreds of scientists involved in this work. And we still do not know with certainty the extent to which declines in songbird populations in North America are due to changes in breeding grounds, wintering grounds, stopover grounds or some combination of the three.

Most birdwatchers who have been out chasing birds for a number of years feel that the migration has lost its splendor, but we also have more rigorous data to back that up. There are a relatively small number of parks in the central Atlantic region where the birds have been censused for several decades now. Those parks show a deep and alarming decline in populations of migratory songbirds and minor changes in populations of nonmigratory songbirds.

What has happened as a result of already lost migrations, like the passenger pigeon or the bison?
I don't think we know what the impacts were because there weren't ecologists around to study these phenomena when they were at their glory. In the intervening years, enough other things have changed to make it difficult to deduce what changes could be attributed to the migration. But it is inconceivable to me that the loss of the most abundant bird on the face of the earth and the greatest mammal migration did not profoundly change the ecosystem.

What are the most unappreciated migrations?
Most of us fail to appreciate just how many species are migratory. We think about birds but we overlook everything from salamanders to bighorn sheep. For all the major groups of animals, there are a significant number who migrate. It's a very common strategy.

And the most endangered?
Clearly, there are endangered species who migrate and by definition those are endangered migrations. But the one that springs to mind is the monarch butterfly. It is still abundant. Yet, all the monarchs of eastern North America funnel into a tiny mountain forest in central Mexico and those forests continue to be destroyed despite efforts by the Mexican government to protect them. A very common species could potentially suffer a very swift population decline.

A second example would be the red knot. It winters in Tierra del Fuego and migrates to the Canadian Arctic. En route every spring they stop in Delaware Bay for a few weeks where they gorge themselves on the eggs of horseshoe crabs. The crabs themselves have migrated from the oceans to the bay to spawn.

These birds have to add 50 percent of their body weight in order to make the last leg of their journey to the Canadian Arctic and the population has crashed over the last couple of decades. Horseshoe crabs are being overfished, resulting in insufficient food for the red knots during this stopover.

The common feature of all of these lost or disappearing migrations seems to be some kind of choke point? Is that the root of the problem?

That's right. If all of the individuals of a migratory species congregate in a small area it makes them highly vulnerable to habitat destruction and overexploitation.

On the other hand, it's probably easier to protect those species than it is those that have more diffuse migratory pathways. We know the places we have to protect for red knots, we just have to have the will to do it. For the cerulean warbler, we know a few of them but the bird has a much broader migratory path than the knot and, consequently, it is much more difficult to figure out what are the key areas to protect.

So what can be done?

There are two key ingredients to protecting migratory phenomenon. One: taking proactive conservation measures, not waiting until species are critically endangered. And second, cooperating across borders and boundaries. All these administrative borders and boundaries between states, federal agencies and nations are utterly meaningless to the migrants.

You can't fully protect [migrations] by individual people behaving better. There has to be a coordinated program to save migratory species. In one respect, the most important thing people can do is elect leaders who care about the environment and who will work to achieve the more comprehensive approach to conservation.

Having said that, there are steps individuals can take that will help migratory animals: One is coffee. Coffee can be grown two different ways: as a monoculture of coffee plants out in the open or as shade-grown coffee where the coffee plants are grown under a canopy of native tropical trees. The shade-grown coffee provides good wintering habitat for many North American songbirds. The coffee monocultures are abysmal habitat. So by drinking shade-grown coffee and creating a bigger market for that product one could help songbirds.

A second one is seafood. A major cause of mortality in migratory seabirds and sea turtles is longline fishing. The animals take the bait, drown or get caught on the hooks and die. So avoiding seafood that is captured on longlines potentially helps migratory animals, too.

Given that some of the solutions require cooperating across borders or inconveniencing landowners by taking down fences, how realistic is it to think that anything can be done?

Let me put it this way: The social skills we are going to need to solve the problem of climate change are precisely the same social skills we'll need to save migrations and vice versa. These sorts of problems have a certain universality to them. They require cooperation and they require people to act before a problem becomes a crisis.

If we're serious about tackling climate change, we can certainly protect animal migrations. And if we become good at protecting animal migration, it will only help us tackle tougher issues like climate change.