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 prevented 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 the 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 per year. But why? Branco and his advisers in both the United States and Brazil suspected that the birds were contaminated by a specific heavy metal that was prevalent in their wintering home in South America.

So when Branco was grounded due to canceled flights, physiological ecologist C. Loren Buck of Northern Arizona University called other scientists on campus to help. Citizen scientists from Wisconsin and Virginia, along with scientists from Florida, collected feathers and shipped them to Arizona. Buck’s team conducted tests on the tail feathers of purple aircraft, where both pollutants and hormones tend to accumulate. They also looked at notes on 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 really meant. The results did not surprise him.

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

The higher the level of mercury found in birds, the lower their fat score. In your study published last December in environmental pollution, Branco points out that the concentration of mercury found in birds could be what is negatively affecting their ability to accumulate fat. The birds likely pick up the mercury in their winter home in the Amazon basin. After flying to North America, the heavy metal in their bodies likely renders them unable to store fat, leaving them without enough energy to migrate south each year. Even a small increase in the amount of the heavy metal in birds, which are part of the swallow family, is likely to result in poor health and a decreased chance of survival.

The Amazon basin is known as a hot spot for mercury contamination. Natural inorganic mercury is washed from mercury beds in the Andes. But the methylmercury that has begun to accumulate in the region thanks to human efforts is more dangerous. Methylmercury is very sticky and gets trapped in animal tissues, making it extremely difficult to remove, Buck says.

The more than 100 hydroelectric projects built in the Amazon in the last 50 years have allowed methylmercury levels to increase in fish and humans. Dams built for these projects slow the flow of water and allow naturally occurring inorganic mercury to settle to the bottom of river beds. There, microbes convert it to methylmercury, the most toxic form of the heavy metal, which can move 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, contaminating water, soil, and the plants and animals that depend on them.

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

Fat drop could affect the breeding ability of purple planes. If breeding occurs late in North America, the resulting chicks may not be mature enough to make the journey south in time. This problem would add to the fact that the adults probably do not have enough energy to fly south. If they attempted the journey, they would probably die before reaching their destination.

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 stage also occurs. When that water is loaded with mercury, the toxic substance is transmitted to insects and any animals that consume them, including purple planes. But the chain of contamination does not end with the birds.

“These elements, such as mercury, should be expected to travel up the food chain,” says Mario Cohn-Haft, an ornithologist and bird curator at the National Institute for Amazon Research, who was not involved in the study. “So anything purple swifts eat is probably also accumulating mercury, even if it didn’t eat anything else that came out of the water.”

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

Although most studies on mercury contamination look at toxicity in humans and fish, Buck and Cohn-Haft agree that more studies of other species are likely to follow. Purple Swifts were chosen as the starting point because they are easy to track. Year after year, they return to breed in the same human-made nest boxes in North America, where they are captured and sampled before being promptly released.

The decline in purple planes is part of a larger problem. “Biodiversity is a great indicator of the health of the planet for everyone, including humans,” Buck says. “When we start to see things like a 50 percent decrease in bird populations in half a century or less, that should send up red flags, and we should really try to understand what that means.”

He says humans must end artisanal gold mining and stop deforestation in the Amazon, which leaves the soil bare and allows mercury-laden runoff to seep into rivers. Hundreds of new hydroelectric 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 Butantan Institute focused on human health who oversaw Branco’s work in Brazil, understanding animal and ecosystem health is directly related to improving conditions for humans. Her work and that of her colleagues focuses on how people, animals, plants and their environment are connected.

“We see ourselves as different, as a species that is not affected by the changes that we ourselves are promoting on earth,” he says. “But it’s a big mistake that could cost us our very existence.”

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