Thousands of migratory swallows settle on an island in the Amazon for 2.5 months : Revista Pesquisa Fapesp

Every day from late January to mid-April, at sunset, thousands of blue swallows (suffered prognosis) arrive from all sides, from high or close to the surface of the river, form a dense cloud, rotate quickly for a few minutes in an apparently disorganized movement, then they all fall at almost the same time and lodge in the trees of Comaru Island, in Iranduba, municipality of almost 50 thousand inhabitants 30 kilometers (km) from Manaus, in the state of Amazonas. And there they spend the night, amid annual trips between North America, where they breed, and South America, where they gain weight by eating insects. In early March, a group of eight biologists from the National Institute for Research in the Amazon (Inpa) decided to count the bird.

For two days, before dawn, Mario Cohn-Haft set out with his team in two boats, each carrying two pairs of researchers. At the ends of the island, each pair looked one way: north or south. One of the members of the pair took notes and the other counted the number of birds that took off every minute and spread through the forest. “Usually it was 20 or 30 per second, but sometimes much more. I couldn’t believe I could see 500 swallows coming out of the trees at once”, says biologist Eliene Fontes Arruda, who took care of the count.

Multiplying it by the time it took for the birds to take off successively – from 30 to 40 minutes, from dawn to 6:30 am – biologists arrived at an estimate of 244,000 swallows that occupied the island the night before. It was a daily average a little lower than that of the beginning of 2020, just before the beginning of the pandemic, when Arruda, applying the method for the first time, estimated the birds in 266 thousand individuals. “The number of birds seems to vary each night, sometimes with more arrivals than departures,” notes Cohn-Haft.

Flock of Blue Swallows over Comaru Island

Jessica Andrade de Oliveira / Inpa

On this year’s trip, the biologists reinforced the conclusion that there is indeed a rotation of swallows on the island, a hypothesis raised on the group’s first trip, in 2019. Arruda placed radio transmitters in about 30 birds, which, added to the 20 installed in 2019 , indicated that the animals use the island as a roost between 2 and 12 days – most of them for a week.

“Blue swallows come out of different parts of the eastern United States and Canada, usually when winter starts, and then they go back there,” she says. “The accumulation of fat in the birds that we captured to examine and measure indicates that they would already be ready to return to North America.”

Considering this turnover, biologists estimate that nearly 3 million blue terns will sleep on Comaru Island by the end of April, when they return to the north. According to Cohn-Haft, the 3 million correspond to a third of the world’s population of blue terns, estimated at 9 million.

Brian E. SmallAn adult male Blue Tern flying in Brazoria County, Southeast Texas, United StatesBrian E. Small

tons of guano
Back in the trees, after feeding during the day, the swallows defecate one to three times a night. Biologist Jessica Andrade de Oliveira, also from INPA, calculated that, every dawn, thousands of birds left 15 to 43 kilograms of nitrogen-rich guano on the trees at that time of year semi-submerged, because of the annual flood on the island of 3 hectares (1 hectare is equivalent to 10 thousand square meters).

“The continuous discharge of guano into the water changes the local fish community,” she notes. On the island, at this time of year, covered by water from the annual flood, and in the surroundings, carnivorous and omnivorous species predominate, such as black piranhas (Serrasalmus spp.), orana (Hemiodus immaculatus) and cangati (Auchenipterichthys longimanus), with rare insectivores and frugivores, as in other parts of the Rio Negro.

According to Oliveira’s analysis, the guano deposited on the island’s soil has a nutrient content three times higher than that of the river banks and up to 15 times higher than that of Papagaio Island, 3 km away, than the swallows. used as a dormitory until a few years ago and was abandoned perhaps because of the excess of large boats with tourists. In the 1990s, says Cohn-Haft, thousands of swallows occupied sheds at an oil refinery in operation in Manaus, which they left in the morning after covering pipes and wires with renewed layers of guano, requiring continuous washing.

“The swallows form a unique environment for 2.5 months on Comaru Island, with an intense production of guano, which changes the local conditions of life of aquatic organisms”, he comments. “The way they form clouds above the trees and then plummet is a form of defense against predators like the peregrine falcon (peregrine falcon), which can be even faster than them.”

Jessica Andrade de Oliveira / InpaOne of Inpa’s research boats approaches Comaru IslandJessica Andrade de Oliveira / Inpa

According to Cohn-Haft, 18 species of swallows fly across Brazil, all characterized by long, pointed wings and a generally forked tail; 10 of them move from one region to another within the country, six arrive from North America and the Caribbean and two from southern South America. “The Southern Swallow (Progne Elegance) is almost identical to the blue swallow, but migrates from Argentina to the Amazon during the southern winter, from May to September, thus overlapping with the blue one for a few months,” he says.

With up to 22 centimeters in length and 55 grams in weight, the blue swallow is the largest species found in Brazil. It focuses on the Amazon, but has been seen throughout the country, even in Rio Grande do Sul. According to an article published in November 2021 in the scientific journal Research in Ornithologythe oldest record of this species was made in 1816 in Manaus. They arrive in Brazil in greater numbers in July, when the temperature begins to drop in the Northern Hemisphere, and at the beginning of the year they gather on the island of the Rio Negro, before returning to the north.

In March, biologist Erika Hingst-Zaher, from the Butantan Institute, accompanied the Inpa team and, with her team, collected two tail feathers and samples of cotton swab from the mouth and cloaca of 100 terns and 30 terns that visited the island of Rio Negro.

The feathers will be used to analyze the levels of mercury, a contaminant disseminated in the soil, water, fish and in the bodies of Amazon residents. According to her, preliminary analyzes carried out by biologist Jonathan Branco, from the University of São Paulo (USP), indicated that the levels of mercury found in these birds are high enough to harm their reproduction and could be linked to the decline in the populations of this species, observed the last 30 years in North America.

The material collected by cotton swab will be used to search for disease-causing viruses. “As they are migratory species, swallows are potential vectors of pathogens between the Northern and Southern hemispheres,” says Hingst-Zaher. This work is part of an epidemiological surveillance network on wild animals funded by the Ministry of Science, Technology and Innovation (MCTI). The results of the analyzes should come out in the coming months.

Scientific articles
SANTOS, CO and others. Distribution and migration phenology of purple martins (suffered prognosis) in Brazil. Research in Ornithology. v. 29, pg. 213-22. november 2021


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