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Each year, hundreds of thousands of tiny Cerulean Warblers depart their summer homes throughout the eastern United States, fly across the Gulf of Mexico, and roost in the high canopies of South America for the winter. Then they come back, to breed and raise their young. Except when they don’t.

Cerulean populations are declining faster than any other warbler species in the United States, and biologists are trying to track the birds throughout their ranges to understand why. But the very act of tracking them might hamper their ability to return to their breeding grounds, according to a in the American Ornithological Society journal The Condor.

“Often people don’t publish results like this, but this is something you should at least take into consideration when trying to decide how to spend your time and resources,” says Than Boves, an ornithologist at Arkansas State University who studies warblers and co-authored the paper. “And if you are trying to conserve a small population—if you have 10 birds in your state—you might not want to do this type of [tracking research], because you might end up with fewer birds than you started out with.”

Scientists who study small migrating birds can’t exactly slap a GPS tag on their subjects because such devices can weigh more than the animals themselves. Many researchers have instead turned to geolocators, which record light levels over time and can determine a bird’s latitude and longitude based on the times of sunrise and sunset. Geolocators are small and lightweight, so while they are much less precise than a GPS tracker or a radio ID tag, they can be the best way to monitor the journeys of tiny, long-distance migrants like warblers.

Geolocators have been used and Blackpoll Warblers. But other studies have suggested the tracking devices can harm individual birds by weighing them down, changing where they go to feed, and altering their migration routes. A  suggested the tags can increase drag on avian bodies, making it harder for them to fly.

Debating the Effect

To get data from a geolocator, field biologists have to recapture the birds. That’s what Boves and graduate student Doug Raybuck set out to do with the Ceruleans. Over the course of two seasons, they captured 101 birds from northwestern Pennsylvania and the Ozarks of Arkansas and Missouri. Some were fitted with geolocators and others were banded as controls. When the researchers went back the next year, they found that 16 percent of geolocator-tagged birds had returned, compared to 35 percent of control birds.

The study’s broader goal is to monitor ceruleans in their breeding grounds and winter ranges, attempt to find their migration routes, and figure out where they might be running into problems, says Boves. “But we also think it’s important, while we’re trying to get this basic information, to account for whether these devices have negative impacts,” he says.

Henry Streby, an evolutionary biologist at the University of Toledo who also studies warblers and geolocators, points out that the devices themselves might not be the problem. Instead of being affected by the geolocators’ weight, warblers could be struggling with the different harnesses used to attach them, he says.

“We have had birds that carried relatively enormous amounts of weight compared to what you think a bird could carry and did their migration perfectly fine,” he says, referencing a 2015 study on Golden-winged Warblers.

Biologists have a couple ways to wrangle a warbler and attach a geolocator to its back. They can use a leg band and crimp it onto the bird, or they can use a backpack-style harness that loops over a bird’s legs. Streby designed the latter method and says it's more effective than the leg band, which he pointed out can cause chafing, irritation, and weight-distribution problems if it slides around. The meta-analysis on tracking tags backs up his claim, finding that the leg band method was more detrimental than the backpack harness.

To attach their geolocators, Raybuck and Boves used leg bands the first year and backpack harnesses the second year. “Because the first year was such a huge negative impact, and they averaged the two years together, they are saying it is a geolocator effect. And it is not. It is a harness effect,” Streby says. He wrote a response to the Condor paper and is waiting to hear about its submission status, he says. Boves and Raybuck, for their part, contend the difference between the two attachment methods was slight.

Weighing the Risk

Cerulean Warblers tagged with the geolocators might have had a few different reasons for failing to come back. They could have simply cut their voyages short, choosing a different breeding ground than the previous year. Or they might have taken too long to return to their roosts, arriving to find their old hangouts occupied by other birds. They might also have died, possibly as a consequence of added weight and drag. Boves and Raybuck couldn’t distinguish among these explanations—they just knew fewer birds came back.

But the multitude of explanations raises some confounding questions even for the birds that did return: Are birds shifting migration patterns because of environmental causes such as deforestation and climate change? Or were they just looking for shortcuts because they’re carrying an extra burden?

Boves says he can’t be sure, but that his team does have preliminary data that suggest the geolocators don’t have much impact on overall travel for the returning birds. Those that made the roundtrip had also spent time in known Cerulean habitat in Peru and Bolivia, he says. They hadn’t taken any shortcuts.

“Those birds passed over a lot of good overwintering habitat to end up in the southern Andes,” he says. “But there’s no way to know for sure, unless we can find out where the control birds went.”

The returning birds brought back other valuable data as well, including information on migration timing and the connection between breeding and wintering habitats. And so Boves and Raybuck say they will continue using geolocators, and that other ornithologists should, too—with caution.

“We are trying to increase the population’s size,” Boves says. “If we can find important parts of the wintering grounds, it’s going to have an impact on many, many more birds than the potential number that we would impact using these techniques.”

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The Cerulean Warbler is just one of many birds that face habitat loss. To help these birds and other wildlife, .