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The Ferruginous Pygmy-Owl is a fierce, fluffy handful of a bird. Plump and voracious, it brings death from above to lizards and mice. But to hawks and larger owls, the tiny raptor is a tempting snack itself. As a result, the owls stay close to the ground, which in turn subjects them to an unusual threat: barriers along the U.S.-Mexico border.

“Everybody thinks a bird’s just going to fly over a fence,” says Aaron Flesch, a biologist at the University of Arizona’s Desert Lab, “but based on their behavior, we just haven't seen much evidence that [these owls are] going to.”  According to Flesch’s research,  Ferruginous Pygmy-Owls fly, on average, just over four and a half feet above the ground; existing border barriers, by contrast, are at least 18 feet high. New walls proposed by the Trump administration may top out above 30 feet. 

Although it may be obvious how a border wall blocks the natural movement of wildlife such as jaguars, tortoises, and bighorn sheep, its effects on animals with wings are less clear. Low-flying pygmy-owls challenge our assumptions about how birds move across a landscape. What’s more, a fence reshapes the topography of a place in subtle ways beyond the structure itself: It’s not that birds can’t fly over it—it’s that, for one reason or another, they often won’t. Instead, stuck in smaller, sparser patches of habitat, with nowhere else to go, these isolated populations can suffer. 

For avian wildlife, the stakes are high. The border region, stretching across four U.S. states and the Rio Grande, is incredibly biodiverse, the result of an overlapping Venn Diagram of ecosystems: tropical, temperate, mountain, wetland, coastal, desert. Hundreds of species of birds call the region home, some already threatened or endangered. It’s also criss-crossed by major migration corridors, which means that changes in this relatively narrow geographic area may be felt as far away as the Arctic.

“It’s not a local thing,” Flesch says. “We’re disturbing a hundred meters around the wall—but the effects are much, much larger. When you disrupt movement of critters across the wall, then you potentially influence these populations at really large scales.”

Of the nearly 2,000 miles of the United States’ border with Mexico, nearly 40 percent—about 650 miles—already has some kind of barrier on it. Starting in San Diego in the 1990s, various presidents have added border fencing in California, Arizona, and Texas. In 2005, the Real ID Act allowed Homeland Security to waive the legal protections of environmental laws like the Endangered Species Act and the Clean Water Act along the southern border, while 2006’s Secure Fence Act turned up the tempo on construction with a plan to fence 700 miles. Trump’s proposals to complete at least 1000 miles of wall would nearly double the amount of physical barriers cutting through these ecosystems. For birds at the border, or even those just passing through, here's how that might play out. 

Range Restriction

After observing hundreds of these tiny birds moving through patches of wilderness on Arizona’s southern border , Flesch witnessed a pygmy-owl flying higher than the tree canopy “only once." Plus, they will often react to wide open spaces—like, say, the gaps cleared for access roads on either side of a border wall—by flying in the other direction. The existing barriers risk cutting pygmy-owls off from moving and breeding, says Flesch, isolating the small U.S. population from the rest of the species in Mexico, and perhaps driving them from the country altogether.

Other ground-dwelling birds are likely to have a similar relationship with a wall, he says, including the relatively common Wild Turkeys and Greater Roadrunners, as well as rarer species like the Northern Bobwhite, Scaled Quail, and the quirky Plain Chachalaca, a tropical bird with a distinctive call and a range just jutting into the southern tip of Texas. Clumsy fliers with a preference for low, brushy cover, all these birds are able to fly short distances, but mostly to escape a predator or find a safe roost. For them, the open area might be even more of a deterrent than the wall itself. 

Habitat Loss

“We’ve definitely already seen the impacts of the wall,” says Nicole Gillett, a conservation advocate at , where just over 200 miles of pedestrian fencing and vehicle barriers already run along the Arizona border. “For birds, the problem is not just the fence. It’s everything that comes along with the fence.”

Starting with its construction, a border wall brings noise, heavy machinery, and newly bulldozed forest and patches of brush. In deserts, the tall stands of trees lining streambanks offer a rare kind of habitat for threatened Gray Hawks and Bald Eagles, with a view of the surrounding expanse. These scrubby groves might not look impressive, but they provide services that exist almost nowhere else in the desert. In the 1990s, a found only three breeding sites for Gray Hawks in the lower Rio Grande Valley.

And while there’s an arbitrary quality to which habitat a wall imperils—this stand of trees, but not that one—any loss of habitat can threaten stressed-out populations. Scientists refer to the “carrying capacity” of a landscape as the number of animals that can comfortably subsist on its natural resources. Carrying capacity can depend on the quantity and quality of habitat, and whether it is connected to other pieces of habitat.

Ernest Herrera, a biologist in Texas’ Rio Grande valley, says that so little natural habitat remains along his stretch of the border—only between 2 and 10 percent of the former extent—that whatever’s left is already at carrying capacity. Here, the Northern Aplomado Falcon, a federally listed endangered species, finds some of its last breeding grounds in the U.S. “Cutting any of that habitat will leave animals without shelter, without food or nesting places," Herrera says. "They’ll be displaced,”

Migration

For migratory species, these same overburdened patches of habitat offer critical opportunities to refuel in the middle of epic journeys. If these rest stops can’t support these passers-through, Herrera says, that will have a long-term effect on populations of birds that spend most of their lives in the Canadian Arctic or the Amazon .

Bright lights like the stadium floodlights positioned along sections of the border can also harms birds’ ability to navigate. In some places along the border, high-wattage security floodlights etch a luminous line east to west across the nighttime landscape, a glittering wall of light that these north-south travelers must cross. “When we disrupt that by putting a light in their way, they’re going to see it as a guiding light,” says Gillett. That could lead them astray or require they spend unnecessary energy to find their way again.

Climate Risks

Climate change magnifies many of the border wall’s risks to birds, and vice versa, Gillett says. Take water. In the Tijuana Estuary, a protected salt marsh along California’s border, the amount and distribution of rainfall, as well as the tides that send pulses of seawater into the wetland twice a day, the exact mix of fresh and saltwater. Under changing climate conditions, the fragile balance in which the estuary thrives .

Here, construction from border fencing also in the mid-2000s, and today culverts carry sediment and trash into the estuary. This sediment can clog up the shallow channels that act as the capillaries of a wetland, turning its lush inlets into pools of standing water—and in the rainy season that means more flooding, too. Threatened Ridgway's Rails that nest in the marsh, as well the threatened Snowy Plovers that forage along the mudflats and marsh grasses, all feel the effects.

On a larger scale, as water scarcity and heat waves stress border region ecosystems, species may move north in search of cooler temperatures. The more fragmented the landscape, the worse a chance they have at finding their way to better habitat. Gillett says many impacts of the border walls will be hard to foresee: “There’s just so much that is still unknown. We’re not going to be able to know the impacts until it’s too late.”

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