Why Are Purple Martins Declining in the United States?

Mercury contamination in their Amazonian wintering grounds may play a role

Purple Martins
Purple martins perch on a branch in the Brazilian Amazon. Ramiro Dário Melinski

Brazilian ecologist Jonathan Maycol Branco had a problem. Unlike the migratory birds he was studying, he couldn’t fly north.

In 2020, the Covid-19 pandemic had hindered the University of São Paulo graduate student’s plans to head to Northern Arizona University to complete his master’s thesis on the eastern North American subspecies of purple martin. The migratory bird, which molts in the Amazon Basin and then flies north to breed in North America, has declined over the past five decades, at a rate of almost 1 percent a year. But why? Branco and his advisors in both the United States and Brazil suspected the birds were being contaminated by a specific heavy metal prevalent in their wintering home in South America.

So when Branco was grounded by canceled flights, physiological ecologist C. Loren Buck, of Northern Arizona University, called in other scientists who were on campus to help. Citizen scientists in Wisconsin and Virginia, along with scientists in Florida, had collected feathers and sent them to Arizona. Buck’s team ran tests on the purple martins’ tail feathers, where both contaminants and hormones tend to accumulate. They also looked at notes about the birds’ body conditions, including their mass and fat. Once they had the raw data in hand, they sent it to Branco in Brazil. He looked at the numbers and developed statistical models to determine what they actually meant. The results didn’t surprise him.

“We expected to find mercury in the feathers,” Branco says. “But what was most striking was the correlation between the level of mercury in the purple martins and their fat score.”

The higher the level of mercury found in the birds, the lower their fat score. In his study published this past December in Environmental Pollution, Branco notes that the concentration of mercury found in the birds could be what is negatively impacting their ability to accumulate fat. The birds likely pick up the mercury in their winter home in the Amazon Basin. After they fly up to North America, the heavy metal in their bodies likely makes them unable to store fat, leaving them without enough energy to migrate south every year. Even a small increase in the amount of the heavy metal in the birds, which are part of the swallow family, likely leads to poor health and a decreased chance of survival.

Purple Martin Being Held
Graduate student Clarissa Oliveira holds a purple martin captured along the Rio Negro in Brazil. The bird was soon released. Erika Hingst-Zaher

The Amazon Basin is known as a hot spot for mercury contamination. Natural inorganic mercury washes down from mercury beds in the Andes. But methylmercury that has started to accumulate in the region thanks to human endeavors is more dangerous. Methylmercury is very sticky and gets caught up in the tissues of animals, making it extremely difficult for them to eliminate, says Buck.

The more than 100 hydroelectric projects constructed in the Amazon over the last 50 years have enabled methylmercury levels in fish and humans to rise. Dams built for these projects slow down water flow and allow natural inorganic mercury to settle on the bottom of riverbeds. There, microbes convert it into methylmercury, the most toxic form of the heavy metal, which can make its way up the food chain and cause neurological and behavioral disorders in animals and humans. Symptoms in people include headaches, insomnia, memory loss, tremors, neuromuscular effects and cognitive and motor dysfunction.

Artisanal miners in the region also use methylmercury to separate gold from other substances, which contaminates water, soil, and the plants and animals that depend on them.

While Branco says more studies need to be done to confirm mercury as a cause of the drop in fat in the birds, he does call what he found very concerning. “The correlation between the two—mercury contamination and fat loss—is extremely negative.”

The drop in fat could affect when purple martins are able to reproduce. If reproduction happens late in North America, the resulting chicks may not be mature enough to make the trip south on time. This problem would be in addition to adults likely not having enough energy to fly south. If they were to attempt the journey, they would likely die before reaching their destination.

The mercury contamination results from the bird’s diet. Mosquitoes and other larger insects tend to lay their eggs in water, where much of the larval phase also occurs. When that water is heavy with mercury, the toxic substance is passed on to the insects and any animals that consume them, including purple martins. But the chain of contamination doesn’t stop with the birds.

“It should be expected that these elements, like mercury, will travel up the food chain,” says Mario Cohn-Haft, an ornithologist and bird curator at the National Institute of Amazonian Research, who was not involved in the study. “So anything that eats purple martins is likely to accumulate mercury as well, even if it didn’t eat anything else that came out of the water.”

Predators that eat the banana-sized birds include peregrine falcons, stygian owls and arboreal snakes.

Though most studies about mercury contamination look at toxicity in humans and fish, Buck and Cohn-Haft agree that more studies of other species are likely to follow this one. Purple martins were chosen as a starting point because they are easy to trace. Year after year, they return to breed in the same human-made nest boxes in North America, where they are grabbed and sampled before being immediately released.

Purple Martin With Band
In the Rio Negro region of the Amazon, researcher Ramiro Dário Melinski holds one of the few purple martins with a numbered leg band from the United States. Erika Hingst-Zaher

The decline of purple martins is part of a bigger problem. “Biodiversity is a huge indicator of planet health for everybody, including humans,” says Buck. “When we start seeing things like a 50 percent decline in avian populations in a half a century or less, that should send up red flags, and we should really try to understand what that means.”

He says humans need to do away with artisanal gold mining and to put a stop to deforestation in the Amazon, which leaves ground bare and allows mercury-laden runoff to seep into rivers. Hundreds of new hydro dams are already planned for the Amazon Basin, he says, which need to be reconsidered before they, too, start trapping mercury in their reservoirs.

For Erika Hingst-Zaher, a biologist at the human health-focused Butantan Institute who oversaw Branco’s work in Brazil, understanding animal and ecosystem health is directly linked to bettering conditions for humans. Her work and that of her colleagues focuses on how people, animals, plants and their environment are all connected.

“We see ourselves as different, as a species that isn’t affected by changes that we ourselves are promoting on the earth,” she says. “But it’s a huge mistake that could cost us our existence.”

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