Blue swallows intrigue scientists as they make a small island in the Amazon their home

On a tiny islet in the middle of the Negro River, in the state of Amazonas, a team of scientists on two speedboats is intently examining the sky. Known as Ilha do Comaru, it is submerged, as it happens every year in March, and only the tops of the trees show above the surface.

A group of blue terns zooms past, cutting through the heavy, humid air of the region, and just above the boats and the island, a flock of black dots begins to accumulate, like clouds of birds.

They then form a synchronized whirlpool. A few minutes later, however, they fall like a rain of black hail, encrusting the trees, while their sound intensifies and occupies the entire beginning of the night.

In a matter of minutes, this spectacle ends, leaving the sky still again.

The island of just five hectares —almost the size of the Morumbi Stadium— attracts a huge number of these swallows with sparkling down. Since it receives visits from approximately 250,000 individuals from February to April, it is considered one of the largest refuges of the species ever discovered.

The role this site plays in the bird’s migration intrigues scientists. Comaru could be the starting point, the researchers suspect, for many of the 9.3 million Great Blue Terns that make their way from South America to North America annually.

Mario Cohn-Haft, curator of birds at Inpa (National Institute for Research in the Amazon), in Manaus, also believes that the island could be fundamental for understanding the constant decline of the species. “It’s the biggest window we have into what blue swallows do in South America,” he says.

The North American breeding population of these birds is estimated to have declined by 25% since 1966. In some areas, the drop has been even more pronounced. In others, the birds have disappeared altogether.

Little is known about the challenges the species encounters when traveling. “If we can track their movements, find out what they are eating and analyze whether they have been contaminated by pesticides and other pollutants, we can learn something about how they are doing here,” he says.

Cohn-Haft, together with American and Brazilian scientists, carried out in 2022 the most comprehensive study ever carried out on Comaru, in search of information that could help guarantee the future of the species.

In North America, the bird’s breeding range extends from Canada to Mexico, but is mainly concentrated east of the Rocky Mountains, in the United States.

Blue swallows nest exclusively in structures humans erect to receive them—from hollow gourds to miniature “condominiums.” Birds often return to the same yard, and even the same structure, every year, which facilitates research with the aid of tracking devices, which must be retrieved along with the valuable information they bring.

If the birds arrive a little late, “our phone rings nonstop with people freaking out, worried that their babies haven’t come back,” says Joe Siegrist, president of the Purple Martin Conservation Association, a nonprofit organization that protects and researches the species in the United States. USA.

But this relationship was not always this close. Natural cavities, such as hollow spaces in trees, were once colonies of blue swallows. Their loss, however — along with competition from aggressive, non-native species like the European starling — has made the birds “100 percent dependent on humans providing housing for them to breed,” says Siegrist.

Although they were once studied extensively in North America, scientific knowledge about blue swallows wanes as they fly south. Exactly where they go, what routes they take and what habitat they encounter along the way remain mysteries.

Blue swallows, being aerial insectivores (they hunt flying insects), are among the fastest declining bird groups, as insect populations have also declined due to the use of pesticides, among other causes. The risk for these birds is further compounded by their very long migration journeys.

They have been sighted for a long time in the Amazon, but the first deeper studies began a few years ago. In 2007, Bridget Stutchbury, a biologist at York University in Toronto, Canada, equipped the first songbirds (those with harmonious songs) — 20 blue terns and 14 wood thrushes — with geolocators.

Stutchbury recovered two swallow geolocators and found that one of them spent the summer (in the southern hemisphere) in the Amazon, while another stayed further south in Brazil for the season.

Seven years later, Kevin Fraser, a postdoctoral fellow supervised by Stutchbury, now an ornithologist at the University of Manitoba, in Canada, equipped 105 blue terns with more advanced trackers.

The 14 datasets he retrieved revealed that all but one had spent the summer in the Amazon and that five had spent a lot of time near Manaus, crammed into an area no bigger than a residential lot.

Siegrist and Fraser, who have research partnerships, decided it was time to take a closer look. They and two other colleagues visited Manaus in November 2016 and 2018 and looked for the locations Fraser’s birds visited, but found nothing.

Later, however, they would understand that they chose the wrong time — they were three months early. They also noticed that the birds had not returned to the island where Fraser had located them. “They switched just when we thought we knew where they were,” says Siegrist.

Finally, in early 2019, Cohn-Haft learned from a student of a huge flock of birds that a local guide had shown her just 30 km from Manaus.

The guide, José Francisco dos Santos de Moraes, began to take some visitors to see the great flock that arrived at dusk. Cohn-Haft, upon going to the scene, knew immediately that they were blue swallows.

“It was one of those absolutely chilling, emotional, inspiring moments”, he recalls, who immediately also imagined that the place could be a treasure trove of scientific data on the species.

Paulão’s Floating Restaurant then became a kind of HQ for research. Anchored close to Comaru Island, it was rented by scientists to serve as a cafeteria, dormitory and field laboratory, as it is easy to find blue swallows in the surroundings for studies. “You can row a boat and pick them off the trees like apples,” says Siegrist.

The group of ornithologists collected samples of material from the birds that will help determine which pathogens they carry — a disease could be the explanation for the decline in the population of blue swallows.

This is the third year that Cohn-Haft and Siegrist have been studying birds in Comaru. They captured about a hundred birds at a time, and most of them seem ready to head to the Northern Hemisphere — with new feathers, better for flying, beefed up muscles and stores of fat for the journey.

Radio tags fitted to birds in past seasons, detected by the scientists’ network of receivers, showed that they stay at most two weeks. This gives Cohn-Haft confidence that many more individuals pass through the region than the count has so far estimated. For him, the number should be closer to 1.5 million.

The fact that such a large fraction of the world’s terns depend on a single location raises concerns. Although there are currently no plans to build a dam on the Rio Negro, a future hydroelectric plant, for example, could flood the island that serves as its roost. “This population density makes them vulnerable,” explains Siegrist.

Research at Comaru can also point to more global conservation concerns. Mercury from natural and human sources, such as mining activities, can travel through the food chain until it reaches swallows. Therefore, the researchers are also investigating how contamination by this heavy metal affects the birds’ endocrine systems, reducing fat reserves and making them less able to migrate.

Scientists hope that whatever discoveries they make will help to understand what is behind the decline of other songbirds, especially other aerial insectivores.

It’s almost midnight when the researchers finish processing the last bird in the laboratory set up on the floating restaurant. Two graduate students cautiously board a boat carrying the animals in cloth bags.

Cohn-Haft, anxious, asks them to hurry. Birds should return to their roost as soon as possible to get a good night’s rest—after all, they have a long journey ahead of them.

What does a blue swallow look like?

• Scientific name: progne subis
• Where do you live? It breeds in the United States, Mexico and southern Canada, but spends the rest of its time in Brazil
• How do you look? As an adult, it is about 20 cm long and weighs between 45 g and 60 g. The wingspan, on average, is 40 cm. Despite its name, depending on the light in which it is observed, it may not look blue, but purple or even green.
• What does it feed on? Insects, usually caught in flight

_{This story was supported by the Pulitzer Prize. Center.}