What’s Killing Migratory Birds? A New Tracking System May Offer Clues.

Red knots in their breeding plumage feed at Golfo de Santa Clara, Mexico. Julián Garcia Walther/Undark

This story was originally published by Undark and appears here as part of the Climate Desk collaboration. 

Twice a year, members of a subspecies of red knots—salmon-colored sandpipers—migrate thousands of miles between their wintering grounds in northern Mexico and breeding sites in the Arctic tundra, encountering myriad obstacles along the way. Thought to migrate during both day and night, brightly lit cities likely disrupt their nighttime journeys, and rising sea levels and invasive species threaten the wetlands they rely on for refueling at stopover sites.

The red knot is one of some 350 North American bird species that migrate. Yet there remains much to learn about the details of their journeys. It’s a critical information gap given the loss of an estimated 3 billion birds in North America since 1970, according to a 2019 study.

“The only way to think about conservation of migratory birds is to consider their full annual cycles,” including their migration routes and wintering sites, said Bill DeLuca, a senior migration ecologist with the National Audubon Society.

The problem, he said, is “We don’t know, for a lot of species, what time of the year is causing the declines.” For the vast majority of migrating birds, the full picture of their life cycle is incomplete, DeLuca added.

That’s partly due to technology. Until recently, while scientists could study birds at their North American breeding sites, they had few ways to track them individually throughout their migrations or while in their wintering grounds, especially small songbirds like warblers and sparrows.

And for birds that migrate through the West’s remote deserts and mountains and across its wild shorelines, like the rufous hummingbird, which journeys between Alaska and the Pacific Northwest and Mexico, their flight routes are even less understood. “Knowledge of migration patterns for birds in the West is way behind the East,” said Mary Whitfield, research director at the California nonprofit Southern Sierra Research Station, because of the smaller number of long-term banding stations there.

But scientists across the West are increasingly turning to an accessible, low-cost technology to answer key questions about bird migration and how climate change is impacting their life cycles.

The Motus Wildlife Tracking System, launched in 2014, is an international network of about 1,800 radio receiver stations in 34 countries. The program, run by the conservation organization Birds Canada, is already well established in eastern North America, but has begun to spread rapidly across the West in the last couple of years. Researchers in the Motus network track birds (or other animals, like butterflies) using small tags.

When a bird flies within range of a station—up to about 12 miles away, depending on the conditions—the tag automatically transmits a signal to a receiver, which is then uploaded to the Motus website. Scientists participate through tagging, building Motus stations, or both, and fund their own projects. Museums, zoos, and schools may also participate by hosting a Motus station and educating the public about bird migration and movement, Whitfield noted. So far, more than 43,600 animals, including butterflies, bats, and birds, have been tagged by researchers using Motus globally.

Until recently, tracking tags were too large and heavy for small songbirds. The Motus system uses tags that weigh less than 3 percent of a bird’s weight—in the case of a small songbird that weighs around 18 grams, a tag weighs just half a gram. After birds are captured in mist nets made of fine mesh, they are fitted with the tags using a harness, which they wear like a backpack.

An estimated 1 billion birds use the Pacific flyway, a route through Western coastal states, during their migration, and many millions more migrate via the central flyway through the interior West. Along the way, they routinely encounter natural phenomena like storms, drought, and predators, as well as man-made obstacles like glass-facades that attract birds and pose serious collision risks. In addition, given the rapid growth of wind and solar projects across the West, Whitfield said, it’s crucial to identify birds’ movements through desert areas earmarked for alternative energy development.

According to Whitfield, Motus (Latin for motion) could be a “game changer” for understanding Western birds’ movements through the seasons. “It’s critical,” Whitfield said. “We have to find out more about migration, because it’s definitely a pinch point for bird mortality—that’s typically when birds die the most, because it’s just a really perilous journey.”

In May of this year at the Bosque del Apache Wildlife Refuge in New Mexico, Matt Webb, an avian ecologist with the Bird Conservancy of the Rockies, was getting ready to install a Motus radio tower with funding from the U.S. Fish and Wildlife Service. He hoped to fill “in some of the knowledge gaps” about grassland songbirds, which are experiencing rapid declines in population. Four species in particular have declined more than 70 percent since 1970, according to the bird conservation network Partners in Flight.

Grassland birds range from the prairies of Saskatchewan to the southernmost edges of the Chihuahuan desert in Mexico. “We’ve got this massive geography that we need to cover adequately” to understand their migration, Webb said.

And the birds don’t just travel during migration, he added—they roam widely during both the breeding season and winter, making them even more difficult to monitor. With data from Motus, Webb said, they hope to “unravel some of those mysteries of why they’re moving around and where they’re going during those seasons.”

Webb was equipped with several long antennas and a shoebox-sized, solar-powered sensor station computer with cellular connectivity for receiving and transmitting data. But the road to the tower site was flooded, after increased snowpack drove high flows in the Rio Grande River.

So Webb and Kylie Lamoree, another Bird Conservancy ecologist, turned to Plan B, surveying old water and communications towers as potential locations. In order to detect tagged birds up to 12 miles away, “We need to get it up above the topography and the vegetation nearby,” Webb said. (He later noted that they were able to go back at the end of August and install the station.)

At the northern end of the Chihuahuan desert, Bosque del Apache National Wildlife Refuge is a major destination for migrating and wintering waterfowl as well as for birders. Webb was seeking to determine if the four grassland birds he’s studying —thick-billed longspurs, chestnut-colored longspurs, Baird’s sparrows, and Sprague’s pipits—are using the refuge during the winter, during migration, or both.

Those four species are small songbirds with ochre, tan, or black plumage that make them well-camouflaged in shortgrass prairie habitat. The birds are difficult to capture for tagging without large vegetation to conceal the researchers’ mist nets, Webb said.

Even so, Webb said the payoff is great: “There’s really never been a technology that works well enough to be able to collect this data” for such tiny birds, he said. And after a bird is tagged with its transmitter “backpack,” it doesn’t need to be recaptured.

Migrating shorebirds are another group of Western birds with steep population losses in recent decades. Julián Garcia Walther, a Mexican biologist and Ph.D. student at the University of Massachusetts, Amherst, is monitoring shorebirds in northwest Mexico to find out more about climate change impacts on sea level rise and biodiversity. “I started thinking about how these birds that live on the interface between land and sea, the intertidal zone, how they’re going to be affected by sea level rise,” Garcia Walther said.

He learned about Motus in 2019, and realized the small tags used in the network were ideal for monitoring red knots, many of which winter in the coastal wetlands of northwest Mexico and whose populations are under pressure. But there were no Motus stations in the region.

Garcia Walther has now installed about 25 Motus stations with the help of the Mexican conservation organization Pronatura Noroeste, where he is the Motus network coordinator, along with other partner organizations. “It’s a big learning curve,” he said, requiring skills in electricity, radio communications, and construction. One of his biggest challenges is sourcing materials in Mexico, so he turned to improvised materials, like a pole once used for an osprey nest converted into an antenna mast.

Another hurdle was capturing the birds. Without tagged birds, stations are “just poles and antennas,” Garcia Walther said. Shorebirds are especially tricky to capture because they disperse across the coastline’s open expanses. While the harness method used for tagging grassland birds is also often used in shorebird research, Garcia Walther added, his team uses glue to secure the tags to the backs of red knots, meaning the birds will shed the devices when they molt.

But with three years of data from some 100 birds, Garcia’s team has made some significant observations. One finding, the result of data from Motus stations as well as GPS loggers—trackers that show fine-scale movements—revealed that during high spring tides, red knots use dried seagrass as rafts to rest on while the tidelands are inundated.

“This is analogous to what’s going to happen with sea-level rise,” Garcia Walther said. The data he has collected should help wildlife researchers plan for the future when there will likely be little shoreline available for roosting, he said, informing strategies to protect, restore, and improve vulnerable habitats.

Garcia Walther said he got advice from colleagues in the US when he was setting up his stations, and he now helps scientists elsewhere in Latin America with their Motus projects.

Blake Barbaree, a senior ecologist at Point Blue Conservation Science with projects in California’s Central Valley, also depends on cross-border collaboration. His team is investigating the impact of drought on shorebirds, using Motus to track the movements of birds in California during the winter as well as during migration.

Since they’re only in the second season, Barbaree said it’s too soon to draw any definitive conclusions, though data collected at Motus towers has confirmed high connectivity between the Central Valley and coastal Washington, as well as the Copper River Delta in Alaska. “Numerous detections at Motus stations along the coasts of Oregon and British Columbia,” he wrote in a follow-up email, “have also highlighted the fact that a network of stopover sites is critical to their migration.”

This linkage, Barbaree said, helps researchers “piece together puzzles of population increases or decreases,” looking for impacts not just in wintering or breeding grounds but in key stopover habitats.

The network “has really opened up a world of migratory connectivity research” on other small animals like insects and bats, Barbaree added. And he’s seen it inspire collaboration between researchers investigating not just birds, but other migratory species.

Motus projects include studies on bats and insects, for example, with more than 340 species tagged to date. And scientists are turning to Motus for help identifying threats common to birds and bats. In 2023, a team from the U.S. Geological Survey installed two coastal Motus stations in California—with plans to install about two dozen more—to monitor three seabird species and three species of bats, to determine potential impacts of offshore energy.

After a major effort last winter to tag grassland birds in northern Mexico, Webb followed their migration north in the spring—via data their tags uploaded to the Motus website. A Baird’s sparrow his team tagged was tracked from Chihuahua to northern Kansas and up through North Dakota and Montana, the first time they had connected migratory stops through North American grassland habitats in such detail. It was “a lot of fun this spring watching the stations every morning,” he said.

DeLuca of the Audubon Society said understanding the life cycles of different species is the first step in revealing the factors causing their decline, like habitat loss or pollution. “When you think of all of the drivers that are pushing these species” towards extinction, he said, “it’s really kind of mind boggling.”

And climate change, he said, is an additional “huge over-arching pressure,” since it affects bird migration directly with impacts like increased severe weather, and indirectly when food resources like fruit or insects aren’t available.

Identifying the habitats birds rely on during migration and winter is key, DeLuca said.

And the Motus network can amplify those efforts.