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We've all seen it: A dog—head low, tongue out, ears pointed forward—intently trots toward a resting flock of birds before breaking into a flat-out sprint. The birds scatter, darting into the sky where they swirl about before eventually settling down again. Undeterred, the dog repeats the chase until it gets tired or its owner decides it's time to go

Mike Weston, an avid Australian birder, has lost count of how many times he’s seen this exact scenario play out. , who is also a researcher on wildlife interactions with humans at Deakin University in Victoria, says that when he mentions something to the dog owners, many "will say to me 'Oh they're just having fun!' And it's like, well, you know what? One of them is having fun. The other one is escaping for their life!"

This is no exaggeration. Since the late 1990s, Weston has researched the impact that domestic dogs have on wildlife—particularly on coastal birds in Australia, like Hooded Plovers and Little Penguins—and has found that the presence of canines can dramatically alter the survival chances of birds. Weston’s research isn’t alone. A vast body of evidence from around the world shows that dogs—especially when they are allowed to roam freely unleashed—have a significant impact on wild birds, mainly when they are breeding, nesting, and migrating. 

Because humans and domestic dogs have had such a close relationship for so long, people often forget, or do not fully realize, that their pets still play the role of predator when in nature, says Weston. Not only do dogs retain many of their ancestral instincts to hunt and kill—never mind what tendencies have been bred into them—but birds and other wildlife perceive them as a wild threat. The fact that dogs are so abundant only exacerbates the problem: In the U.S. alone, for example, there are more than 78 million dogs, according to the American Society for the Prevention of Cruelty to Animals.

Owning dogs as pets or taking them outside isn't the problem, Weston is careful to note, but dog owners often "disconnect from non-domesticated animals and their needs who are also sharing the world with us." And even if owners are aware of the impacts dogs can have on wildlife, they often don't think their own dog is the problem, he says. "In my experience, they often have a view that it is other people’s dogs that are the issue.'"

In many public spaces, the standard solution to preventing dogs from disturbing wildlife and habitat has been to create designated areas where they can run and play freely. These are often closed-off dog runs, but some parks and beaches also have special off-leash zones and hours. Others might require leashes at all times or in certain protected areas for wildlife. But these rules and regulations—typically indicated by signage—are regularly flouted by dog owners and seldom enforced by authorities. When that happens, the results can be disastrous for birds and their young. 

Off-leash dogs have perhaps the biggest impact on nesting birds, especially on beaches. Some dogs might actually try to catch and eat adult birds or their chicks, but even a curious dog can create a deadly situation. That's the case on the beaches of Southwest Florida, where Adam DiNuovo coordinates ����APP Florida’s , which monitors around 16 species of shorebirds and seabirds in that area.

DiNuovo says that in the area that he manages, Wilson's Plovers, Snowy Plovers, and Black Skimmers nest directly on the beach during the summer. An unleashed dog that goes near or actively disturbs a colony can cause the whole group to abandon their nests, either for their own safety or in an attempt to lure the threat away. Meanwhile, the eggs and chicks are left exposed, putting them at risk of overheating in the summer sun or of predation, either by the dog itself or another species, like an opportunistic crow. “They can’t fly, and they can’t get away,” DiNuovo says of young chicks. “When an unleashed dog is on the beach, it can directly harm them.”

Weston’s own research shows just how grave a danger canines can pose to chicks.  he co-authored, which focused on 17.4 miles of Australian coast, found that when dogs were banned from beaches where birds nested, the survival rates of chicks went up from 12 percent to 40 percent. It's no wonder: Another  on California’s Santa Barbara beaches showed that, "while leashed dogs disturbed 11 percent of all birds, unleashed dogs disturbed 34 percent of them." 

The problem is not exclusive to coastal habitats. In 2003, another research team that in England’s dreary Snake Pass moors, unleashed dogs increased the chances of chicks dying before fledging by 23 percent. And in the early 1990s, researchers studying the impacts that dogs had on 36 nests of Golden Plovers at Snake Pass that, during incubation, the nesting birds flushed quicker when a human walking nearby was accompanied by a dog. Once the chicks hatched, the adult birds spent 15 percent more energy than usual when dogs were around.

This speaks to another, less obvious issue with dogs in the outdoors: Even if birds are regularly exposed to them, they don't habituate to their presence. The unpredictable movements and speed of dogs leave birds anxious, says Weston. For them, a life already full of natural threats becomes all about "watching out for danger, making decisions about when to respond properly to danger, and surviving encounters that might be potentially lethal." 

An increased perception of danger may lead birds to avoid habitats entirely. In northern Sydney, for instance, woodland trails where people walked with leashed dogs had 35 percent less bird biodiversity than the trails where dog walking was forbidden,in 2007. Meanwhile, people who hiked the woods without dogs had half the impact on birds than those walking with their pets. 

In busy city parks, vital refuges for birds and other urban wildlife, dogs still have the impact of a top predator, says , director of NYC Parks Wildlife Unit. Unleashed dogs wandering unrestricted can harass wildlife, destroy nests, disperse food sources, and even erase paths used by other animals, Simon says. “The problem with dogs is more of a problem with people,” he says. “What your dog is doing is following its instinct. The dogs don't understand the rules, people do.”

Unleashed dogs can be especially stressful for migrating birds, says Kaitlyn Parkins, a senior conservation biologist at . In many coastal cities, like New York, migrants rely on green spaces as stopover sites along their long journeys. Central Park hosts more than 250 species of birds, says Parkins, and in The Ramble, a protected area in the park that requires leashes, migrants can find running fresh water, a wide variety of food sources, and cool understory habitat as they prepare for the next leg of their arduous trek. The mere scent of dogs “could put on alert a bird that is resting, that needs to refuel and to save energy,” Parkins says.

For Weston, getting dog owners to understand the dangers their pets pose so that they follow leash laws remains the biggest challenge. Leash regulations face fierce  from , who often argue their pets need to exercise to justify leaving them unleashed, he says. In 2014, after reviewing 133 papers about the impacts of dogs in parks, researchers that on average, 63 percent of the dog owners didn't follow the rules in places with leash requirements. And yet,  found that dog owners "were more likely to feel obliged to leash their dog when they believed other people expected dogs to be leashed, and when they believed their dog was a threat to wildlife or people."

In other words, the culture around leashing needs to change, and Weston remains optimistic that can happen. In just the past three decades, dog-walking communities have radically changed their behavior toward picking up dog feces, he points out. "That's been a major cultural change,” Weston says. “So if you can change those behaviors in people, there is hope to do it in terms of leashing."