America’s birds are in trouble. Since 1970, nearly 3 billion birds have vanished from the skies over North America.

Most of those losses have been in migratory species, which may breed in the United States or Canada in the summer before heading elsewhere for the winter. Many spend more time living on Caribbean beaches or in Costa Rican forests than they do in American backyards. “They’re really visitors to North America,” said Viviana Ruiz-Gutierrez, co-director of the Center for Avian Population Studies at the Cornell Lab of Ornithology.

Protecting these birds will require working across international borders and safeguarding all of their habitats, many of which are under threat. If migrating birds lose their winter refuges, the consequences will ripple across the hemisphere.

“If we lose Central America’s forests, we can lose North America’s birds,” said Jeremy Radachowsky, the director for Mesoamerica and the western Caribbean at the Wildlife Conservation Society.

To illuminate these connections, scientists at the Cornell Lab of Ornithology developed “shared stewardship” maps in collaboration with Partners in Flight, an international bird conservation network. Each map displays the key wintering grounds for the migratory species that have a significant summer presence in a particular U.S. state or region. The maps are based on data from eBird, a database of observations from bird watchers around the world.

Here are some of those connections.

The New York Metro Area

Many of New York City’s avian tourists summer on the shore and then migrate to winter habitats along the coasts of the southern United States or the Caribbean. Numerous shorebird populations have declined steeply in recent years, and they face threats throughout the entire migratory cycle.

“There’s very little land along coasts that is free of disturbance or pollution of one kind or another,” said Ken Rosenberg, a conservation scientist who recently retired from the Cornell lab and helped create Partners in Flight. “Shorebirds are very vulnerable to that kind of disturbance.”

The petite, plump piping plover has been the focus of conservation efforts along its East Coast breeding grounds, including in New York City. But about one-third of the Atlantic population spends the winter in the Bahamas, where many of the birds tend to congregate in just a few locations, scientists have found. “What that means is by doing work in those areas, you can make a large difference on the conservation of that species,” said Deb Hahn, the international relations director at the Association of Fish & Wildlife Agencies.

Ms. Hahn coordinates Southern Wings, which helps state fish and wildlife agencies connect with conservation groups working in the places where their bird populations winter. Through Southern Wings, 41 state agencies have donated nearly $3.9 million to projects in 11 countries, including to piping plover projects in the Bahamas.

The New York metro area is also home to many forest-dwelling songbirds. The wood thrush, whose heralded song is often said to sound like a flute, breeds throughout the eastern United States, but its population has plummeted over the last half century. In the fall, many wood thrushes head to the Maya Forest, which spans Mexico, Guatemala and Belize and is one of five major swaths of intact forest remaining in Central America.

“Some migratory species, including some that are in pretty steep decline, are hugely dependent on only these ‘five great forests,’” said Anna Lello-Smith, the avian conservation coordinator for Mesoamerica and the western Caribbean at the Wildlife Conservation Society.

But cattle ranching, much of it illegal, has resulted in significant deforestation, she said. The Wildlife Conservation Society is now working with a coalition of Central American governments and other organizations, such as the Mesoamerican Alliance for People and Forests, to protect these habitats as part of the Five Great Forests Initiative.

Missouri

Missouri provides breeding habitats for many grassland bird species, which have been faring especially poorly in recent decades. Many spend the winter in the southeastern United States, but others find their way to South America.

The dickcissel, a yellow-breasted songbird, winters in Venezuela, while the striking bobolink — males sport black and white feathers during the breeding season — journeys as far as Argentina, one of the longest migrations for a songbird. Both species, which sometimes feed on crops, have been targeted for being pests. Human development and the expansion of farming also pose risks to grassland species, as does the mowing of fields, where many birds build their nests.

Missouri also hosts many forest birds, which may migrate to Cuba, Central America’s five great forests or the slopes of South America’s Andes Mountains. The cerulean warbler, which can be found breeding in the Appalachian Mountains or the Ozarks, relies on forests in the Andes, which have also been threatened by agriculture and other human activities.

“We can’t ignore the threats that these birds are facing when they’re beyond our borders,” said Sarah Kendrick, a migratory bird biologist for the U.S. Fish and Wildlife Service and a former state ornithologist for the Missouri Department of Conservation.

SELVA, a conservation nonprofit based in Colombia, has been working to learn more about the cerulean warbler’s movements and survival during its time in South America; several Missouri birding groups donated funds to these recent tracking efforts. Through the Neotropical Flyways Project, SELVA and its partners have also identified a site in the Caribbean foothills of Costa Rica that serves as an important refueling site for migrating cerulean warblers. Tree planting and landowner engagement efforts are now underway there. “We have been able to implement conservation recommendations almost immediately,” said Nick Bayly, SELVA’s director of migratory ecology. “That’s been really exciting.”

Colorado

Colorado hosts breeding populations of many Great Plains and Western species, such as the Swainson’s hawk, whose migration is something of a natural spectacle. The raptors travel to the grasslands of Argentina, soaring on currents of warm air in flocks that can contain thousands of birds.

But many Colorado species make much shorter trips, hopping down to Mexico for the winter. That includes the mountain plover, a grassland bird whose tawny-colored coat provides such good camouflage that it has been called the “prairie ghost.” Some of the birds, which are listed as near threatened, winter in the arid grasslands of Northern Mexico. (Others stay stateside, waiting out winter in Texas or California.)

Climate change, overgrazing by local livestock and the expansion of farming all threaten these grasslands, which provide winter habitats for many Great Plains birds. A variety of Mexican nonprofits, community groups and universities are working to monitor, restore and protect these grasslands, often in partnership with U.S.-based groups, such as the Bird Conservancy of the Rockies.

Colorado’s cliffs and caves also provide homes for one of the most mysterious migrating birds: the black swift, which sometimes breeds behind waterfalls. In the fall, some of the birds seem to fly, nonstop, to the Amazon. But black swifts are spotted so rarely in their wintering grounds that the Cornell team cannot include the birds on its stewardship maps. “There’s no birders that see them there,” said Andrew Stillman, a postdoctoral fellow at the Cornell Lab of Ornithology and ​​the Cornell Atkinson Center for Sustainability. “It’s very likely that they just don’t land during the winter.”

California

Hundreds of West Coast species migrate along what is known as the Pacific Flyway, which stretches from Alaska to the tip of South America. Many venture no farther than Mexico, where they tend to stay closer to the Pacific Coast than many of Colorado’s bird species do, said Archie Jiang, a research technician at the Cornell lab.

For instance, the mangrove swamps of the Marismas Nacionales, where conservation efforts are underway, are a popular destination for shorebirds and waterfowl. And many hermit warblers, which breed in the Sierra Nevada and Cascade mountains, spend winter in Mexico’s pine-oak forests. “This is a classic Pacific Northwest to west Mexico species,” Mr. Jiang said.

But some elegant terns — anchovy-loving seabirds that nest in large colonies — fly all the way down to Chile.

Then there is Wilson’s phalarope, an unusual shorebird that often lives inland, despite a predilection for salt water. The birds breed at marshes and wetlands across the Western United States and Canada. Then, many of them assemble at Utah’s Great Salt Lake before flying off to the salt lakes of the high Andes.

How to Help

These maps are just a snapshot of the migratory connections that exist across the hemisphere, and experts highlighted many more species, habitats and conservation projects worthy of attention. There is no single pattern that describes the migratory behavior of birds in California or New York, and protecting these birds will require efforts that are as diverse and far-flung as their habitats.

“All these birds have figured out different ways of doing this big thing called migration,” Dr. Stillman said. “It reflects many, many hundreds of thousands of years of evolution. And it’s neat to see how different birds have arrived at different answers.”

The Cornell lab is making the shared stewardship maps available to state agencies in hopes of helping them determine where they might want to invest in research and conservation. But the scientists also hope that the maps will help the public understand that our migratory birds are shared — as is the responsibility for protecting them.

American bird-lovers can help by supporting conservation efforts abroad, through donations, responsible bird-watching tours or the purchase of “bird-friendly” coffee, which is grown without pesticides on farms that maintain healthy habitats for birds. “It’s critical for the general public to be aware of the choices that they make and how they can actually have a positive impact somewhere else,” said Camila Gómez, director of education and training at SELVA.

We can also become better hosts of these birds while they are in our own backyards. Simple but helpful steps include treating windows to prevent birds from colliding with buildings, keeping cats indoors and planting native greenery, experts said.

“None of us alone can do it,” Dr. Ruiz-Gutierrez said. It is the local, on-the-ground conservation efforts in Central and South America that results in many Americans seeing birds in their backyards in the spring, she added. “And then whatever you do that summer results in them seeing birds returned back in the fall,” she said.” And it’s really that connection that’s important.”