What if migratory animals couldn’t reach their destinations?

Every spring and fall, some animals go on very long trips. Birds fly, butterflies flutter and deer walk.

They need places to rest and eat on the way. What if those places were gone, or a road was in the way?

Migration is a regular journey animals make between places, often between summer and winter homes. Some of these journeys are astonishing: a shorebird called the red knot can fly more than 9,300 miles each spring.

No animal can make such a trip in one go without help from the land. They need stopover places to refuel and safe paths to follow. This thought experiment asks what would happen if migrating animals could not reach their destinations, and what people do about it.

A migratory population relies on several places in sequence: breeding range, nonbreeding range and the stopovers or corridors that connect them. Each must provide food, safety and passage at the right time of year.

This thought experiment removes links in that chain: a stopover site that disappears, or a corridor cut by roads and fences. Real monitoring of shorebirds, butterflies and big-game migrations lets us reason about the consequences and the conservation responses.

Monarch butterflies fly all the way to the mountains of Mexico for the winter. They have never been there before, but they find the way!

Pronghorn and mule deer walk between their summer and winter homes in Wyoming.

Eastern monarch butterflies fly south each fall to the same mountain forests in Mexico, traveling 50 to 100 miles a day for up to two months. The butterflies that make the trip have never been there before.

In western Wyoming, herds of pronghorn and mule deer have walked between their summer and winter ranges for thousands of years, squeezing through a narrow ridge called Trappers Point. And red knots stop in Delaware Bay each spring to feast on horseshoe crab eggs before flying on to the Arctic.

The eastern monarch overwinters in 11 to 12 mountain areas of central Mexico, in oyamel fir forests 2,400 to 3,600 meters up, from October to late March; migrants travel 50–100 miles a day for up to two months, and the generation that arrives has never been there before.

Rufa red knots fly up to 19,000 miles a year. Delaware Bay is the final Atlantic stopover for 50 to 80 percent of the population each spring. At Trappers Point, Wyoming, mule deer and pronghorn have used a sagebrush bottleneck between rivers for millennia.

Imagine a bird's favorite resting beach had no food. Or imagine a busy road across the deer's path.

The animals would have a much harder trip. That is why people work to keep their paths open.

Imagine the red knots' stopover had no horseshoe crab eggs. Red knots rely on those eggs to gain weight quickly before the last long flight, so they might arrive in the Arctic tired and underweight. That part is an idea, but the need for the eggs is a fact.

Now imagine the deer and pronghorn meeting a busy highway with fences. Before wildlife crossings were built at Trappers Point, antelope bunched up between the fence and the road, trying to crawl under and dart across. Many could not get through safely.

Red knots arriving in Delaware Bay depend on horseshoe crab eggs for remarkable rates of weight gain before departing for Arctic breeding grounds. A stopover without that food might leave birds unable to complete the journey in good condition: a reasoned idea, not a measured result here.

Roads and fences can sever corridors. At Trappers Point, biologist Hall Sawyer watched pronghorn bunch between fence and highway during migrations; about 85 vehicle–animal collisions a year once occurred there. Across the United States, drivers hit an estimated 1 to 2 million animals a year.

A long trip is like a long car ride. You need to stop for food and rest.

If one stop is missing, the whole trip gets harder.

Before a long flight, red knots store large amounts of fat as fuel, and their stomach and gut even shrink while their flight muscles grow. They have to refill at each stop. If a stop is missing, the next leg may be too long to manage.

For animals that walk, like pronghorn, a migration is a path across the land. One blocked section, like a fenced highway, can cut off the whole route, even if the summer and winter homes are fine.

Long-distance migrants budget energy across legs: red knots accumulate fat and reorganize organs before departure, shrinking the gizzard, gut and liver while enlarging flight muscles. Their success depends on predictable, high-quality food at staging sites, and they show moderate fidelity to particular flyways and stopovers.

Terrestrial migrants face a connectivity problem: a corridor is only as passable as its narrowest bottleneck. Removing one stopover or blocking one bottleneck can disconnect seasonal ranges that are otherwise intact.

People build bridges just for animals! Deer and pronghorn walk over the road on a bridge covered with dirt and plants.

Mexico also protects the forests where monarch butterflies spend the winter.

In 2012, Wyoming finished two wildlife overpasses, six underpasses and about 12 miles of fencing at Trappers Point. The fences guide animals to the crossings, and the overpasses are covered with dirt and gravel. In the first three years, antelope and deer crossed more than 60,000 times, and collisions fell by more than 80 percent.

Protecting stopovers and wintering places helps too. Mexico created the Monarch Butterfly Biosphere Reserve in 1986 to protect the monarchs' winter forests.

Wyoming DOT's $11 million Trappers Point project (2012) combined two overpasses, six underpasses and some 12 miles of funnel fencing; the Wyoming Game and Fish Department reports more than 60,000 crossings in three years and collisions down by more than 80 percent. In Colorado, similar structures on Highway 9 cut collisions by almost 90 percent by the second winter. A four-lane interstate overpass can cost up to $7 million.

Habitat protection secures the other links: Mexico established the Monarch Butterfly Biosphere Reserve in 1986, and the U.S. Fish and Wildlife Service names Delaware Bay as a key staging area whose food supply red knots depend on.