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Things have been looking up lately for the Great Lakes population of Piping Plovers. This past summer, were the first in the state since the 1950s and the first on Lake Erie since 1977. Michigan’s Sleeping Bear Dunes National Lakeshore boasted 41 breeding pairs in 2017, of 28 pairs. And since they were listed as federally endangered in 1986, the number of plover pairs nesting along the Great Lakes coast has grown from about 17 to 76, the result of work by Լ����Ƶ and others to monitor the birds and protect the beaches where they breed.

Despite this success, the Piping Plover rebound might be in trouble, according to a study . That’s because plovers aren’t the only birds recovering in the region. Like other raptors, Merlins—a small falcon species that regularly preys on plovers and other shorebirds—have been bouncing back since the pesticide DDT was banned in 1972. The new study found that without more aggressive human intervention, an increase in predation by Merlins could significantly hamper the Great Lakes Piping Plover comeback. 

At some plover nesting sites in Michigan, wildlife managers have already turned to predator control—both nonlethal and lethal for a variety of birds and mammals—to keep the recovery program on track. The decision to trap or kill Merlins, which are still considered threatened in Michigan, is not one they take lightly. But if today’s approach to Merlin management goes unchanged, the Great Lakes plover population could shrink to 67 pairs by 2026, according to the statistical model used in the study. Without accounting for the growing Merlin population, the model would have predicted continued recovery to 91 plover pairs during that period. With increased Merlin control, it projected about 117 plover pairs in 2026.

These findings are important for Piping Plover conservation efforts because earlier population modeling didn’t consider the Merlin rebound, says Sarah Saunders, a postdoctoral ecologist at Michigan State University who led the research. “The influence of predators on this population has never been explicitly quantified,” says Saunders, who begins a position as an Լ����Ƶ quantitative ecologist in March. “Being able to, for the first time, get a sense of how predators are affecting the population is new and important for how we can adjust management strategies. The predator community is changing, and therefore our management might need to compensate for that going forward.”

Each breeding season, plover monitors keep careful records of how many adults are at each nest and how many chicks they fledge. Saunders and colleagues fed their model with the monitoring data from every known Great Lakes nest between 1993 and 2016. Other input for the model included observations from two popular raptor-watching sites—Hawk Mountain in Pennsylvania and Whitefish Point in Michigan—for an estimate of the region’s Merlin population.

Their findings indicate that the abundance of Merlins in the Great Lakes region was an important factor in plover survival—an observation that tracks with what wildlife managers are seeing in the field. “I’d say Merlins are the number one predator of adult plovers that we’ve seen in the past 15 to 20 years,” says Vince Cavalieri, Great Lakes Piping Plover Coordinator for the U.S. Fish and Wildlife Service. “We’ve known for quite some time that Merlins are presenting a significant barrier to recovery. But given that they’re a native predator that’s always naturally been here, we’ve been pretty cautious about how we go ahead with control efforts. It’s a very targeted control program, where we’re just looking to remove a handful of Merlins that have the potential or have been preying on plovers at some of our sites.”

It’s difficult to pin down exactly how effective Merlin control is in protecting plovers—there are few nests to judge by, and many other factors involved—but there’s evidence that it works, Cavalieri says. In 2012, North Manitou Island and Ludington State Park, both on Lake Michigan, had similar nesting numbers: North Manitou’s eight breeding plover pairs fledged 11 chicks, while Ludington had seven adult pairs and 12 fledged chicks. In 2016, North Manitou—where there had been lethal and nonlethal Merlin control—20 pairs fledged 46 chicks. At Ludington—where, like at other Michigan state parks, nothing was done to control Merlins—there was just one pair of adult plovers, and no chicks fledged.

Based on the new findings, Cavalieri—who helped secure funding for the study but wasn’t directly involved in it—says he plans to focus on expanding non-lethal measures into nesting areas that haven’t seen any Merlin control. In recent years, he has partnered with a falconry club to allow its members to trap Merlins at some plover nesting areas, and he plans to bring in a government raptor expert to capture more of the birds for removal during Piping Plover breeding season.

Still, trapping a problem Merlin might take weeks, during which it could take out several adult plovers. Killing them is faster, even though it requires Cavalieri to obtain state and federal permits and call in Wildlife Services, part of the U.S. Department of Agriculture, to shoot the birds. Cavalieri says he expects to keep lethal control at or near current levels, typically fewer than 10 Merlins per year.

If it can be done effectively, non-lethal control should be the priority when a protected species is preying on an endangered one, says Gary Roemer, a New Mexico State University wildlife ecologist who has been involved in similar ethical conservation decisions related to a then-endangered island fox in California. With Merlin numbers stable or increasing across North America, Roemer says protecting plovers should be the priority. “Personally, I feel like if the protected species being removed is still doing well in other areas, then I would certainly opt for doing whatever it takes to save the endangered species,” he says.

Keith Russell, a program manager for , offered a similar perspective. The newly repopulated plover nesting sites in his state don’t currently overlap with Merlin breeding range, he says, so controlling the raptors isn’t a concern there. But he noted that Kirtland’s Warbler, an endangered species that nests in Michigan’s jack pine forests, may be extinct if not for intensive trapping and killing of Brown-headed Cowbirds, which lay their eggs in the warbler’s nests, reducing chick survival. “So I suspect that if Merlins threatened the Great Lakes Piping Plover breeding population with imminent extinction, most people would probably agree that removing Merlins could be justified,” he says. “Shy of that dire scenario, if other strategies that might lessen the Merlin’s impacts on the plovers could be devised, they would be preferable.” (Russell also notes that climate change could also affect both birds' ranges in the future.)

The Endangered Species Act doesn’t provide any clear guidance on the ethics of killing one species to protect another, Roemer says. Wildlife managers have to make tough choices on a case-by-case basis. “I’m not envious of the people who have to make these decisions,” he says. “Merlins are such beautiful little birds. I wouldn’t want to be the one killing them, but in some cases you might have to.”